Updated The Travis Config

* Let's run the test on php 5.3.3 too since we are supposed to support it.
* The composer install should take place under `install` rather than `before_script`.
* I've wrapped the composer install in `travis_retry` so that if there is a networking issue causing a failed download, then travis will retry the command upto 3 times before giving up.
* I've removed the `--dev` flag from the composer install since it's obsolte - composer will install dev-dependencies by default, and we don't even have any dev-dependencies anyway.
* I've removed hhvm from `allow_failures` since the test suite has been passing on hhvm for a long time.