Fix extension building on ruby 2.1.0.preview2

Ruby preview 2 will have RubyGems 2.2.0.preview.2, but extension
building tests for ruby versions newer than 2.2.0.  A prerelease is less
than a release, so the import of rubygems into tunk broke this feature.

The restriction has been reduced to 2.2.0.preview.2 to fix the bug.

This imports r43646 from ruby trunk.