Restore monkey patch to Symbol

It pains me immensely to pen a ticket summary like this one, but c'est la vie.

Backstory:

• back in 2011, for http://projects.puppetlabs.com/issues/5466, we added a monkey patch to add a <=> method to Symbol to allow sorting
• in 2012, as part of cleanup the monkey patch was made conditional on whether that version of Ruby already had a <=> method for Symbol. As it turns out, ruby 1.8.7 lacks that method, but ruby 1.9.3+ have that method (but a different implementation than our monkey patch).
• in PUP-4330, we vendored the plist gem into puppet. This gem happens to include much the same patch (but without the conditional logic).
• in PUP-1455 (not yet released), we unvendored the plist gem, thus effectively removing that monkey patch for all rubies currently supported.

This broke one spec test in the puppet-server test suite, which was how this was discovered. However, given how low-level this is, the potential for breakage in modules seems pretty high, so the prudent approach here is to restore the monkey patch.