Gemfile improvements: Optional therubyracer, guard in development group, no direct execjs

* Optional therubyracer allows asset precompilation without bundling a V8 runtime by using Node
* Guard in test group automatically excludes guard when the test group is not being used. The guard docs no longer mention that guard should be put into a tools group.
* No execjs dependency allows production use with precompiled assets without a JavaScript runtime on the production servers