docs: add 2.2.x to the official supported versions

Author: rmehner

README.md

@@ -214,7 +214,7 @@ for the latest [git master](http://rubydoc.info/github/transloadit/ruby-sdk/mast
 
 ## Compatibility
 
-At a minimum, this gem should work on MRI 2.1.0, 2.0.0, 1.9.3, 1.9.2, Rubinius,
+At a minimum, this gem should work on MRI 2.2.0, 2.1.0, 2.0.0, 1.9.3, 1.9.2, Rubinius,
 and JRuby in 1.9 mode. It may also work on older ruby versions, but support for those
 Rubies is not guaranteed. If it doesn't work on one of the officially supported Rubies, please file a
 [bug report](https://github.com/transloadit/ruby-sdk/issues). Compatibility patches for other Rubies