I know this is probably more a Ruby question but I work on several projects where I've used compass plugins. Later on I upgrade these plugins and start new projects. The issue arises when I need to go back and make amends to an older project.
How can I amend my config.rb file to use a specific version of a gem?
For example I often use the Compass plugin: http://rubygems.org/gems/html5-boilerplate/versions
The line in my config.rb file looks like this:
require 'html5-boilerplate'
But I would need 1.0.0 for one project and 2.1.0 in another.
Suggest using rvm (the Ruby enVironment Manager), bundler and gemsets. rvm for managing ruby versions and for allowing you to use gemsets - bundler to specify the versions of the gems you want to use in the project. After you install the bundler gem and have a Gemfile that uses the syntax you have already discovered
gem "html5-boilerplate", "~> 2.1.0"
bundle install will then bring down the gems you need for your project. Read more at http://gembundler.com
gemsets can also be project specific and different gemsets can have the same gems at different versions. you can specify the gemset associated with a project via a .rvmrc file in the project's root.
There are several nice posts out there on how to set up rvm and gemsets. Here's a good example of one to get you going:
http://adamstacoviak.com/posts/super-easy-ruby-gem-management-with-rvm-gemsets/
And here's one that also covers bundler: