Improved travis configuration
Improved travis configuration

* Removed unmaintained Symfony 2.4
* Added Symfony 2.5
* Added test for lowest dependencies
* Removed not needed shell wrapper
* Removed not needed --dev flag
* Used Travis's new docker based env
* Cached composer's deps
Behat/Behat,Taluu/Behat,spolischook/behat,v-abramchuk/Behat,spolischook/behat,spolischook/behat