Add PostgreSQL10 to TravisCI through Docker container.
Use PHP7.2 instead of PHP7.1 as default test environment.
Update to current stable MariaDB 10.2 from stable (GA) 10.1.
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.