Improved travis configuration

* Use faster docker based environment (`sudo: false`)
* Cache the composer dists (that's why `--prefer-source` is removed)
* Include a test for the lowest dependencies (this isn't 100%, as it is checking against 5.4.36, while you claim to support 5.4.0. Unfortuantely, Travis does only have 5.4.36...)
In that case, why not remove the package? We can also do some conditional support, e.g. if installed enable it and otherwise, just ignore it?