Support Rubinius gems binstubs location

Rubinius 2.0 configures RubyGems to install gem binstubs in `PREFIX/gems/bin` instead of just `PREFIX/bin`.

The reason given by evanphx is that Rubinius wants to avoid RubyGems' binstubs conflating with those from other Rubies. http://git.io/XBWGew

Fixes #178
/cc @sstephenson