[CHEF-3933] Add support for Rubygems 2.0

Fixes an issue where chef tries to require a file that's been removed in Rubygems 2.0. After that, there were 4 failing tests, these are resolved as well.

I manually tested `gem_package`, `chef_gem`, and `gem_package` with a source attribute (gem package on disk) and added a regression test for an issue I found.