Fix travis php 5.3 build

https://travis-ci.org/kriswallsmith/assetic/jobs/18190826

This was supposed to be fixed as noted here:
https://github.com/travis-ci/travis-ci/issues/1385

This is mostly a hack for php 5.3 travis builds to use `composer install
--dev --prefer-source` so that builds can be completed.

I tested the build off my fork as well: [![Build Status](https://travis-ci.org/Andrewpk/assetic.png?branch=master)](https://travis-ci.org/Andrewpk/assetic)