Fix handling for ruby version-specific gem paths

It seems that, depending on what version of ruby you have, bundler
will use subtly different paths when gems are bundled locally using
BUNDLE_PATH.

Let's say your gems are bundled in /path/to/project/vendor/bundle
because you set BUNDLE_PATH to ./vendor/bundle in .bundle/config, and
you have BUNDLE_DISABLED_SHARED_GEMS set. If you're on ruby 1.8.x, your
bundled gems will be in /path/to/project/vendor/bundle/ruby/1.8/. On
1.9.x, your gems will be in /path/to/project/vendor/bundle/ruby/1.9.1/.

This change properly detects the correct gem path for rvm (rbenv was
working correctly)
Merge pull request #34 from a-warner/master

Fix handling for ruby version-specific gem paths