[4.3] Upgraded To Symfony 2.6

I know you probably don't want to do this just now, but this pull request is here for whenever you want a one quick upgrade later on. I'll keep it rebased, and travis should kick of tests every time I rebase so we can see if anything breaks.

Also, do we really need to so tightly specify the symfony versions? Laravel isn't too tightly coupled to (most of) the components. Would a `~2.3` on some of the components be more suitable?