travis: Test on zsh 5.6.2

.travis.yml

@@ -11,6 +11,7 @@ env:
   matrix:
     # Use _ZSH_VERSION since if ZSH_VERSION is present, travis cacher thinks it
     # is running in zsh and tries to use zsh specific functions.
+    - _ZSH_VERSION=5.6.2
     - _ZSH_VERSION=5.5.1
     - _ZSH_VERSION=5.4.2
     - _ZSH_VERSION=5.3.1