Symfony 2.3 compatibility fix.

The Response object didn't support the HTTP status code constants in Symfony v2.3.
Are these the only usages of these constants across the lib?

I'll force Symfony ~2.3 on [the example app](https://github.com/skqr/hateoas-bundle-example) so that we can be sure that at the very least the API functional tests it includes pass with the LTS version.