Use Ruby 2.1 instead of 2.1.0

Ruby is now following semver starting with 2.1, and patch versions will use 2.1.1 instead of appending 2.1.0-p1
Remove outdated matrix exclusion
Merge pull request #370 from justincampbell/ruby-2.1

Use Ruby 2.1 instead of 2.1.0 for CI