Update requirements to PHP7 for dev. Add php-mock-phpunit to mock and stub global functions in a better way.
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.
Testing several Travis CI features. Add DBs to travis execution matrix. Doxygen API docu generation and deployment to gh-pages. Update phpunit to 5.7.