Just created a Today Widget using XCODE 6.3.1 and have it working on a MINI running iOS 8.3.
When we try and install it on an iPhone 4 running iOS 7.1, it does not install or run and XCODE reports "Finished Running" with no errors etc.
We will be checking out the log, etc, but wanted to see if anyone else has seen this.
That's because widgets only work on iOS 8 or MacOS 10.10 & newer versions of those OS'es.
Which is documented in the first paragraph of this Apple documentation.