Drop support for ruby ree, 1.9.2

https://www.ruby-lang.org/en/news/2014/07/01/eol-for-1-8-7-and-1-9-2/
remove 1.9.2 gem conditional
test latest version of minor versions

cc @JuanitoFatas https://github.com/jch/html-pipeline/pull/156#discussion_r18380144
add 2.2.x to travis build matrix
Merge pull request #156 from jch/sunset-old-rubies

Drop support for ruby ree, 1.9.2