Changed API documentation deployment to gh_pages to Travis's GitHub Pages
deployment.
Also add Travis GitHub Release Deployment step to offer API
documentation.
Both steps are optional and need to be activated in Travis by setting
GH_TOKEN.
The trusty distro contains MySQL 5.6. Add a precise distro with MySQL 5.5.
Unfortunately 5.7 is not yet provided, which would be interesting
because of the enabled strict SQL mode.
Use newer distro for Travis CI runs.
New environment provides PostgreSQL9.6.
Also no need to install custom Doxygen, made it optional.
Changed default PHP environment from PHP7.0 to current PHP7.1.
Changed codecoverage reporting.
Travis CI has Doxygen 1.7. We need 1.8 to generate our API
documentation. Get a static version and use it.
Always build API Documentation, but changed Doxygen configuration to
only print out errors in the documentation generation, so these can be
reviewed.
