Update CHANGELOG

Author: brucehsu

CHANGELOG.md

@@ -9,6 +9,7 @@
 
 * [#1559](https://github.com/ruby-grape/grape/pull/1559): You can once again pass `nil` to optional attributes with `values` validation set - [@ghiculescu](https://github.com/ghiculescu).
 * [#1562](https://github.com/ruby-grape/grape/pull/1562): Fix rainbow gem installation failure above ruby 2.3.3 on travis-ci - [@brucehsu](https://github.com/brucehsu).
+* [#1561](https://github.com/ruby-grape/grape/pull/1561): Fix performance issue introduced by duplicated calls in StackableValue#[] - [@brucehsu](https://github.com/brucehsu).
 
 ### 0.19.1 (1/9/2017)