Fix mention of C extensions in README

JRuby's extension is part of json now and is written in Java.

Author: headius

README.rdoc

@@ -8,9 +8,9 @@ will be two variants available:
 
 * A pure ruby variant, that relies on the iconv and the stringscan
   extensions, which are both part of the ruby standard library.
-* The quite a bit faster C extension variant, which is in parts implemented
-  in C and comes with its own unicode conversion functions and a parser
-  generated by the ragel state machine compiler
+* The quite a bit faster native extension variant, which is in parts
+  implemented in C or Java and comes with its own unicode conversion
+  functions and a parser generated by the ragel state machine compiler
   http://www.cs.queensu.ca/~thurston/ragel .
 
 Both variants of the JSON generator generate UTF-8 character sequences by