fix travis build errors

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.
Merge branch 'master' of https://github.com/Andrewpk/assetic