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