always update to latest rubygems or attribute to set version

Default to always updating to the latest rubygems unless attribute is set to specific rubygems version. This fixes the issue of fpm failing on Ruby 2.1.0 build nodes. The default version of rubygems 2.2.0 shipped with ruby 2.1.0 and causes the fpm error. Using the latest rubygems version 2.2.1 corrects the error.