[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-3935] Use stripped down lockless logger

Ruby's stdlib Logger wraps all IO in mutexes. Ruby 2.0 doesn't allow you
to request a lock in a trap handler because that could deadlock. This
commit fixes by replacing the Logger with a lock-free variant.