Update .travis.yml

Include:

- Added Symfony 2.3 to the list of dependencies to test. It is out now and it I think it's worth to find out, what is missing to support it officially
- Added PHP 5.4 and 5.5 to the interpreter to test. It doesn't hurt and may be helpful
- Added `"prefer-stable": true` to `composer.json`. Dependencies werent installable otherwise.