(MAINT) Remove PUPPET_LOADED from Gemfile

Remove the use of the PUPPET_LOADED environment variable from
the Gemfile, as the bug requiring its use has been eliminated by
Puppet Server's upgrade to JRuby 1.7.17.
Merge pull request #3420 from fpringvaldsen/remove-PUPPET_LOADED

(MAINT) Remove PUPPET_LOADED from Gemfile