Make sure the Qt version used is >= 5.2.

Not sure if Qt5.3 will be compatible with this, but as I can not find a
pre-packaged Qt5.3 version this is not an issue for the moment.
Remove explicit include path (no longer needed).
Dump versions in the before_script session.
Use Qt5.2 on Travis.
Travis: put the CMake build into the "install" section.