Set GEM_PATH when using built-in gemcache

This allows rubygems to work with gems loaded from lib/gemcache.

Specifically, without this the [`Kernel.gem`](http://ruby-doc.org/stdlib-1.9.2/libdoc/rubygems/rdoc/Kernel.html#method-i-gem) will not activate gems in `lib/gemcache` correctly.

This can cause problems if you manually install certain gems (such as `pg`) to `lib/gemcache` instead of an RVM gemset or system location.