delegate synchronize method

Delegate MonitorMixin#synchronize body to Monitor#synchronize.
It makes guarantee interrupt safe (because Monitor#synchronize is
written in C). I thought Ruby implementation is also safe, but I
got stuck failure <http://ci.rvm.jp/results/trunk_test@P895/2327639>
so that I introduce this fix to guarantee interrupt safe.

ext/monitor/lib/monitor.rb

@@ -205,12 +205,7 @@ module MonitorMixin
   # +MonitorMixin+.
   #
   def mon_synchronize(&b)
-    @mon_data.enter
-    begin
-      yield
-    ensure
-      @mon_data.exit
-    end
+    @mon_data.synchronize(&b)
   end
   alias synchronize mon_synchronize