add specific php version 5.3.3 to travis build to validate issue #84

This leads to a failing build. 

The build is fixed by #85 (tested via the combined branch alexmmm/json-schema@a2982e89534a7f6d2748be0fd8156721a53596b1)

Also added composer install option '--prefer-source',to avoid ssl issues in php 5.3.3.