chore(travis): run tests on PHP 5.3.3 and 5.6

5.3.3 is the lowest supported version, so we should test explicitly on that.

PHP 5.6 isn't stable yet, but we want to be ready for it.
Merge pull request #7078 from ewinslow/travis-versions

chore(travis): run tests on PHP 5.3.3 and 5.6