Set GEM_HOME to Gem.user_dir instead of Gem.paths.home

Changed ChefDK::Helpers.omnibus_env to set GEM_HOME to Gem.user_dir instead of GEM_HOME. Also changed the tests to expect that as well. 

It works for me, but really not clear where Gem.user_dir comes from and if this may cause other people other problems.

Before this change `GEM_HOME` would be set to `/opt/chefdk/embedded/lib/ruby/gems/2.1.0` I wasn't really clear how `Gem.paths.home` ever got set to that. 

After this change `GEM_HOME` gets set to `/Users/rberger/.chefdk/gem/ruby/2.1.0` (Obviously it will be someone elses home on another system)