Skip <tt> tags (EmojiFilter)
Use Linguist lexers
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
Backward compatibility: use Pygments if language not recognized by Linguist
test latest version of minor versions

cc @JuanitoFatas https://github.com/jch/html-pipeline/pull/156#discussion_r18380144
Simplify and rename get_lexer
add 2.2.x to travis build matrix
Merge pull request #153 from pchaigno/linguist-lexers

Use Linguist lexers
Merge pull request #147 from moskvax/emojifilter-skip-tt-tags

<tt> tags are not skipped by EmojiFilter
Merge pull request #156 from jch/sunset-old-rubies

Drop support for ruby ree, 1.9.2