Make gem-src as a rbenv plugin

By making use of the rbenv's hook feature, a load path can be injected to all
rubies under rbenv. This makes gem-src enabled for all gem and bundle commands
invoked through rbenv.

This is effective only if it is installed as a rbenv plugin like described in
README, so it does not disturb the existing users using this as a gem plugin.
Return true in the post_install hook

Even though rubygems library does not see the return values of the post_install
hooks, bundler sees them and outputs the error message when they are the false
value. Return true to avoid this error message.
Merge pull request #7 from draftcode/rbenv_plugin

Make gem-src as a rbenv plugin