Drop support for ruby ree, 1.9.2

End of life for 1.8.7 and 1.9.2 was July 31, 2014. This drops support for those versions. The major version will be bumped to reflect this. This is slated for a 2.0 release.

**edit** this also adds ruby-head to the build matrix, but allows failures so we can prepare for future rubies.