Fix Rubinius gem binstubs path

Rubinius 2 insists that it installs RubyGems binstubs into `PREFIX/gems/bin` instead of `PREFIX/bin`.

This creates complexity for rbenv rehash and exec processes, so we symlink `gems/bin` into `bin` and have RubyGems create binstubs at a location that is consistent with other Ruby implementations.

@sstephenson Right now this is done unconditionally. Should we only be doing this when ruby-build is invoked through rbenv-install? I'm wondering whether this might be unwanted by people who use ruby-build for non rbenv-related purposes.

See sstephenson/rbenv#178, sstephenson/rbenv#461