Adjust `composer.json` & `travis.yml`

Requirements:
- [x] PR Sylius/SyliusResourceBundle#18 must be merged first

-----------------------
`composer.json`:
- add branch alias,
- use latest known versions for external dependencies,
- use same PHP version as Symfony2

`travis.yml`:
- add tests for PHP 5.5,
- use `prefer-source` option to avoid GH limits