[CHEF-3933] use rubygems 2.0 files/classes/methods

* rubygems/format file removed
* Gem::Package replaces functionality in Gem::Format
* Gem.sources returns a SourceList which must be coerced to array
[CHEF-3933] support new gem dependency installer return type

in rubygems commit 432fc4818bbf14dfa5e49bbc1950eaa6d3fde133, return type
of dependency_installer's #find_gems_with_sources is changed to return a
Gem::AvailableSet collection instead of an ordered Array.
[CHEF-3950] Fix wrong variable name in GemEnvironment#spec_from_file

Introduced by fab4a2a and fixed by f67808e5 in 11-stable.