Drop 1.8.7/REE/1.9.2 support

This removes the Nokogiri version lock, which means that 1.8.7/REE is no longer supported. Also, a fresh install grabs an activemodel/activesupport that requires 1.9.3, so 1.9.2 is out as well. That is less of a problem for anyone.

Also, the README has been updated to reflect this. We ran tests on 1.9.3, 2.0.0, and 2.1.0 with no problems. I highly recommend adding the gem to [Travis CI](https://travis-ci.org/) so you can see at a glance what versions do and don't work.