Removed tests for PHP 5.4 and 5.5. We only test for 5.3.3 and 5.6.

As discussed, this PR adds tests for PHP 5.3.3 and 5.6 only.

It takes too much time to have everything run on all PHP versions. We need to make tests faster, this should help a bit.

Waiting for the build to complete on Travis to validate the changes.