Update RUBYGEMS_VERSION to 2.2.2.

http://rubygems.org/pages/download still shows 2.1.11 as the latest version.

There's some discussion about integrating this into the rubygems/rubygems release task in issue #629.
Merge pull request #656 from TimMoore/patch-1

Update RUBYGEMS_VERSION to 2.2.2.