More changes in .travis.yml

Build in Debug mode, to make builds faster.  But set
DEB_DEFAULT to OFF, we do not want to enable these
extra checks.

Enable OCE_TESTING to launch OCE tests.

Now that builds are faster, enable OCE_OCAF and
OCE_DATAEXCHANGE.  As we do not yet install X11
development packages, set OCE_DISABLE_X11 to ON.