Adjust `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

Adjust `travis.yml`:
- add installation of mongodb PHP extension to allow testing `doctrine/mongo-odm`
Merge pull request #18 from stloyd/feature/composer

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