Add Symfony v2.5 to travis

AFAIK there is no differences in dependencies resolution.

I used https://github.com/sonata-project/SonataAdminBundle/blob/master/.travis.yml as an example and compared the Travis output https://travis-ci.org/sonata-project/SonataAdminBundle/jobs/32436971 with https://travis-ci.org/sonata-project/SonataBlockBundle/jobs/32410582

The way the SonataAdminBundle uses seams more efficient. That's why I changed it here.