Spock unit test works but mvn test don't

How come running a unit test in STS works and but mvn test in bash gives produces an error. Groovy Libraries are in the classpath. I'm also curious how mvn test differs from spock test. Can't mvn test pick's up spock test?

[ERROR] Failed to execute goal org.apache.maven.plugins:maven-surefire-plugin:2.18.1:test (default-test) on project purple: Execution default-test of goal org.apache.maven.plugins:maven-surefire-plugin:2.18.1:test failed: There was an error in the forked process
[ERROR] java.lang.NoClassDefFoundError: groovy/lang/GroovyObject
[ERROR]     at java.lang.ClassLoader.defineClass1(Native Method)
[ERROR]     at java.lang.ClassLoader.defineClass(ClassLoader.java:760)
[ERROR]     at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
[ERROR]     at java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
[ERROR]     at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
[ERROR]     at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
[ERROR]     at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
[ERROR]     at java.security.AccessController.doPrivileged(Native Method)
[ERROR]     at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
[ERROR]     at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
[ERROR]     at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
[ERROR]     at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
[ERROR]     at org.apache.maven.surefire.util.DefaultScanResult.loadClass(DefaultScanResult.java:131)
[ERROR]     at org.apache.maven.surefire.util.DefaultScanResult.applyFilter(DefaultScanResult.java:95)
[ERROR]     at org.apache.maven.surefire.junit4.JUnit4Provider.scanClassPath(JUnit4Provider.java:222)
[ERROR]     at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:107)
[ERROR]     at org.apache.maven.surefire.booter.ForkedBooter.invokeProviderInSameClassLoader(ForkedBooter.java:203)
[ERROR]     at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:155)
[ERROR]     at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:103)
[ERROR] Caused by: java.lang.ClassNotFoundException: groovy.lang.GroovyObject
[ERROR]     at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
[ERROR]     at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
[ERROR]     at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
[ERROR]     at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
[ERROR]     ... 19 more
[ERROR] 
[ERROR] -> [Help 1]

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.springframework</groupId>
    <artifactId>purple</artifactId>
    <version>0.1.0</version>

    <properties>
        <java.version>1.8</java.version>
        <gmaven-plugin.version>1.4</gmaven-plugin.version>
        <groovy.version>2.4.0</groovy.version>
        <spock.core.version>1.0-groovy-2.4</spock.core.version>
    </properties>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.5.3.RELEASE</version>
    </parent>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-hateoas</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>
        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
        </dependency>
        <dependency>
            <groupId>com.jayway.jsonpath</groupId>
            <artifactId>json-path</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.spockframework</groupId>
            <artifactId>spock-core</artifactId>
            <version>${spock.core.version}</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
            <plugin>
                <groupId>org.codehaus.gmaven</groupId>
                <artifactId>groovy-maven-plugin</artifactId>
                <version>1.5</version>
                <dependencies>
                    <dependency>
                        <groupId>org.codehaus.groovy</groupId>
                        <artifactId>groovy-all</artifactId>
                        <version>${groovy.version}</version>
                    </dependency>
                </dependencies>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-surefire-plugin</artifactId>
                <configuration>
                    <includes>
                        <include>**/*Test.class</include>
                        <include>**/*Spec.class</include>
                    </includes>
                </configuration>
            </plugin>
        </plugins>
    </build>

    <repositories>
        <repository>
            <id>spring-releases</id>
            <url>https://repo.spring.io/libs-release</url>
        </repository>
    </repositories>
    <pluginRepositories>
        <pluginRepository>
            <id>spring-releases</id>
            <url>https://repo.spring.io/libs-release</url>
        </pluginRepository>
    </pluginRepositories>
</project>

Solution

You should be using the gmaven-plus plugin, also your groovy libraries are quite outdated:

<plugin>
        <!-- The gmavenplus plugin is used to compile Groovy code. To learn more about this plugin, 
        visit https://github.com/groovy/GMavenPlus/wiki -->
        <groupId>org.codehaus.gmavenplus</groupId>
        <artifactId>gmavenplus-plugin</artifactId>
        <version>1.5</version>
        <executions>
          <execution>
            <goals>
              <goal>compile</goal>
              <goal>testCompile</goal>
            </goals>
          </execution>
        </executions>
</plugin>

Checkout the official spock-example project for a working configuration.