Remove external dependencies from build

The `rbenv` module probably shouldn't be responsible for declaring
external dependencies, despite [this commit][1]. The manner in which
these dependencies are managed should be at the user's discretion.

[1]: http://git.io/Zm39Og
Make dependencies in gem test more explicit
Remove Git dependency from plugin

The `rbenv` module probably shouldn't be responsible for declaring
external dependencies like Git (despite [this commit][1]). The manner in
which such dependencies are managed should be at the user's discretion.

[1]: http://git.io/DWCQiw
Merge branch 'fix-gem' into integration
Merge branch 'fix-plugin' into integration
Merge branch 'fix-build' into integration