Fix compilation issue with ruby22.

See https://github.com/flori/json/issues/229

Taken from the ruby-core changes.

--- ext/json/ext/fbuffer/fbuffer.h.orig	2014-12-27 21:27:31.701991610 +0100
+++ ext/json/ext/fbuffer/fbuffer.h	2014-12-27 21:28:11.252956858 +0100
@@ -172,7 +172,7 @@
 
 static VALUE fbuffer_to_s(FBuffer *fb)
 {
-    VALUE result = rb_str_new(FBUFFER_PAIR(fb));
+    VALUE result = rb_str_new(FBUFFER_PTR(fb), FBUFFER_LEN(fb));
     fbuffer_free(fb);
     FORCE_UTF8(result);
     return result;