Update `README.md`; mruby is now Ruby 2.x compatible.
## What is mruby
mruby is the lightweight implementation of the Ruby language complying to (part
-of) the [ISO standard][ISO-standard]. Its syntax is Ruby 1.9 compatible.
+of) the [ISO standard][ISO-standard]. Its syntax is Ruby 2.x compatible.
mruby can be linked and embedded within your application. We provide the
interpreter program "mruby" and the interactive mruby shell "mirb" as examples.
