Update CHANGELOG

Author: stjhimy

CHANGELOG.md

@@ -3,6 +3,7 @@
 
 * [#1216](https://github.com/ruby-grape/grape/pull/1142): Fix JSON error response when calling `error!` with non-Strings - [@jrforrest](https://github.com/jrforrest).
 * [#1225](https://github.com/ruby-grape/grape/pull/1225): Fix `given` with nested params not returning correct declared params - [@JanStevens](https://github.com/JanStevens).
+* [#1227](https://github.com/ruby-grape/grape/pull/1227): Store message_key on validation exception - [@stjhimy](https://github.com/sthimy).
 * Your contribution here.
 
 0.14.0 (12/07/2015)