[4.2] Require The Key To Be 32 Chars On PHP 5.6+

In response to #6722, I propose that we enforce a key length of exactly 32 on laravel 4.2 if we are running php 5.6+, and in laravel 5.0, I propose we then enforce this length, whatever your php version.
Oh, wow. Just come out of writing symfony code style all day...