.travis.yml: Disable testing with openjdk6

All infrastructure I use runs openjdk7, so testing on openjdk6 would be
a waste of resources.
.travis.yml: Correct again for deploying to GitHub

This way only tagged commits get deployed. For now this is good enough,
since I don't need every single commit to be deployed to the web server.
Every single commit on master still gets integrated and tested on
Travis, though.