Travis-CI: Refactor .travis.yml and document

This further simplifies and cleans up the Travis-CI file without making
any major functional changes.

With this structure .travis.yml is much easier to read and thanks to the
added documentation next developers are less likely to mess it up again
as they can more easily see how the jobs matrix work.

Number of jobs:
- before: 33 (with 7 ignored)
- after:  33 (with 7 ignored)

This partially reverts commits c12609dd and 193bc89a.

Remove unnecessary dependencies that nothing uses: dpatch libzmq-dev

Install extra build dependencies from a PPA specifically tailored for
Travis-CI use and which ensures all architectures are supported (in this
case s390x is needed and was not available from mariadb.org repos).

Validated with https://config.travis-ci.com/explore