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...)
But if the package is always installed with core, what is the sense of a seperate package?