[wip] Update integration testing stuff

This is still not working as expected (data integration test not udpated yet).
Skip tests against PostgreSQL for the moment
Minor change: variable renamed in a test
Ensure that MySQL and PostgreSQL are used
Remove indirect proof and disable MySQL tests

[ci skip]
Remove obsolete classes.
Fix failing test case.
Fix typo in comment.
Use MySQL for integration tests.
Remove HSQL from integration tests.
Update .travis.yml
Integrate Cobertura to coveralls.io
Give a try to JaCoCo