Integrate with Travis-CI for easier and more automatic QA

I simplified and refactored the original suggestion from Daniel Black regarding a .travis.yml file that enables the usage of the continous integration service by Travis, which is free for open source projects and easily available to anybody who wants to developd and work with the MariaDB server sources.

This Travis-CI file is not intended to replace the buildbots currently used by MariaDB core developers, but rather to provide an extra facility for occasional contributors who don't have their own buildbot trees.

Currently Travis-CI has a 50 minute limit on the build duration, so the tests are limited to the 'main' suite. Parallel Travis-CI jobs would be a way around that restriction, but it wastes resources, so I am working with Travis-CI dev to figure out another more effective solution.

The Travis build also includes building the Debian packages, which will help spot packaging breakages early on.

The code in this PR branch passes the test. Log visible at https://travis-ci.org/MariaDB/server/builds/92879843