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

Requirements:
- [ ] PR Sylius/SyliusResourceBundle#18 must be merged first

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`composer.json`:
- add branch alias,
- remove `minimum-stability` to avoid BC breaks with external packages,
- 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