buildRubyGem: use Gem.use_paths to load gems

After ruby initializes, rubygems no longer reads the GEM_PATH. Before,
we have the following scenario:

    Gem.path # => ["a"]
    ENV['GEM_PATH'] = ["b"]
    Gem.path # => ["a"] # Still returns the same

Gem.use_paths is the documented way to create isolated environments as
documented in [1].

[1] http://www.rubydoc.info/github/rubygems/rubygems/Gem.use_paths