Add commented framework.validation.cache = apc to prod config
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
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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
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2bfdaa0 Add commented framework.validation.cache = apc to prod config
cleaning
kernel ok
layout ok
common config
config dev
config prod
config test
default routing
dev routing
bootstrap
readme
updating readme
updating readme
updating readme
forget package name for packagist
doc for installing