Allow using older symfony components for PHP 5.4
Allow using older symfony components for PHP 5.4

Symfony 3.0 components have a platform requirement to PHP >= 5.5.
This change uses the `--prefer-lowest` flag in combination with composer update
to be able to build on travis with PHP 5.4.

deployphp/deployer,robfrawley/deployer,mbrodala/deployer,oanhnn/deployer,jverdeyen/deployer,cjsewell/deployer,prosalov/deployer,torinaki/deployer,robfrawley/deployer,prosalov/deployer,TheRatG/deployer,deployphp/deployer,deployphp/deployer