Use latest rubies
Merge pull request #44 from sue445/feature/ruby2.1.3

Use latest rubies
:golf:

Ruby "2.2" now points to 2.2.0-preview1, and "2.1" to 2.1.3 on .org. If you want to test older version, say "2.1.2" specifically.
https://twitter.com/travisci/statuses/513233940442644480
Merge pull request #45 from sue445/feature/tweak_travis

:golf:
current parser is not supported ruby 2.2
Write changelog [ci skip]
Merge pull request #46 from sue445/feature/ruby_2.2

Support ruby 2.2
Tweak doc [ci skip]