merged branch adrienbrault/patch-1 (PR #520)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #520).

Discussion
----------

Add commented framework.validation.cache = apc to prod config

Hey,

After some XHProf, I found that this setting was important to get the best performance in production.

I think it make sense to have it as a comment like doctrine caches.

Commits
-------

2bfdaa0 Add commented framework.validation.cache = apc to prod config

Author: fabpot

app/config/config_prod.yml

@@ -8,6 +8,8 @@ imports:
 framework:
     router:
         strict_requirements: null
+    #validation:
+    #    cache: apc
 
 #doctrine:
 #    orm: