Update Travis Test Configuration: Travis no longer supports PHP 5.3.2

As discussed in https://github.com/travis-ci/travis-ci/issues/516#issuecomment-5858003 and implemented in https://github.com/travis-ci/travis-cookbooks/commit/a65154fa570259318ae8a819ae427bad5bbd4ace - the list of PHP versions available on Travis is now: "5.2.17", "5.3.3", "5.3.13", "5.4.3".

In our .travis.yml we should be using:

php

  - 5.2

  - 5.3.3

  - 5.3

  - 5.4

On develop 5.2 should be removed.

We should also consider dropping support for 5.3.2 while dropping support for versions < 5.3.2 in 3.1, as newer Symfony versions require 5.3.3 now since all major linux distributions ship with a version of 5.3.3.