Travis CI: Add PHP 5.5 and use more build defaults

**About Travis wrong build errors:**

**Attention:** At the moment, there is an issue on travis-ci.org side, that make RMT builds to wrongly fail (see travis-ci/travis-ci#1188). 

I'd guess that other PHP projects that rely on [@expectedException](https://github.com/liip/RMT/blob/master/test/Liip/RMT/Tests/Unit/VCS/GitTest.php#L82) annotation could also be be impacted...

**About these insignificant proposed changes:**

Beside adding `5.5` to the build matrix, other changes are just cosmetics (aiming to simplify `.travis.yml`). Let me know if you:
* prefer to still use `composer` from http://getcomposer.org/installer instead of preinstalled version.
* still want to explicitly declare `script: phpunit` 

Note: `install: composer install ...` as default [was originally planned](https://github.com/travis-ci/travis-build/blob/master/lib/travis/build/script/php.rb#L26). I would like to know if such feature would be appreciated on your side...