changing gem install to use chef_gem

I'm working to migrate over to the omnibus installer for chef.  After switching, the rvm::system recipe was failing with the following error:

FATAL: LoadError: no such file to load -- rvm

I'm not exactly sure what is going on, but I think it has something to do with the omnibus rubygems and system rubygems being different.  Switching the gem install to chef_gem fixed the problem for me.  Again, I don't really understand the details of the omnibus packaging yet, so I'm not sure if this change is "safe" or not for people using the recipe under different circumstances.