travis: use slim generic image, and some fixups

Now that caches are distinct (https://github.com/travis-ci/travis-ci/issues/4393),
we can use the Travis minimal image.
The minimal image should take less time to setup and lead to quicker builds.

Also addressed while I'm in here:

• No need to delete the broken google-chrome repo in the minimal image
• Set the hostname to work-around an openjdk bug
• Remove the non-functional apt-cache option
• Remove useless message at completion
• Install jre where the java tests are run