Adjust `composer.json`:
- add branch alias,
- use latest known versions for external dependencies (if possible),
- use same PHP version as Symfony2

Adjust `travis.yml`:
- add tests for PHP 5.5,
- use `prefer-source` option to avoid GH limits
Merge pull request #24 from stloyd/feature/composer_travis

Adjust `composer.json` & `travis.yml`
Replace `CartOperator` with event listeners