Fix compatibility with ruby 2.0

Gem.available? was removed from Ruby 2.0, so we need to check to see if the 'find_all_by_name' method exists and use that if it doesnt. If not, just use the old Gem.available? since we must be on a version of ruby that still uses that. 

I ran rake test, but peachy_namespace_example.rb blows up with the latest retxml/Nokogiri gems, so there is work to be done on the namespace front.