changed dependencies to allow all subsequent Symfony 2.x releases

I'd suggest it should be possible now to say that this bundle should now be able to support all remaining Symfony 2.x releases - this change specifies that (and allows use of the 2.3.0 version of symfony/assetic-bundle which has already been tagged). It also removes the direct specification of kriswallsmith/assetic, letting the specification fall back to that given by the symfony/assetic-bundle package.