Switching Travis CI builds to Ubuntu Bionic Beaver
Switching Travis CI builds to Ubuntu Bionic Beaver

According to the [documentation](https://docs.travis-ci.com/user/reference/trusty/#container-based-infrastructure), `sudo: false` is no longed needed since Trusty.

BenMorel/dbal,drieschel/dbal,BenMorel/dbal,drieschel/dbal