Updated composer.json and .travis.yml to make 2.5.* tests work again

The Symfony 2.5.* tests used to fail because the composer installation timed out. This has been fixed now, basically by requiring Symfony >= 2.5.5 (somthing must have been messed up with earlier versions' dependencies). I also moved all those doctrine packages to the require-dev section instead of always including them separately.

Can you also please add the 2.1.0 stable version? This would be very helpful for component inclusion in other projects.