Update ruby version compatibility in document
Now, Oj has been supported ruby 2.4 or later.
**Ruby**
-Oj is compatible with Ruby 2.0.0, 2.1, 2.2, 2.3, 2.4 and RBX.
+Oj is compatible with Ruby 2.4+ and RBX.
Support for JRuby has been removed as JRuby no longer supports C extensions and
there are bugs in the older versions that are not being fixed.
