Fix travis ci

Split slow tests into batches so they take under 30 minutes each batch.
More than that and travis will time them out.

Disable openjdk6 because it isn't recommended.

And add travis icon to the readme.

Important stuff:

Each job has about 2 minutes and 45 seconds of overhead on travis which
is mostly taken up downloading stuff with maven and compiling code.  So
it is best to have as few jobs as you can without them going over 30 minutes
each.

When travis jobs timeout they just report "error" and often don't log
why the error happened.

This configuration will take up about 2 hours 20 minutes of build executor
time per push including pull requests.  That is about half an hour per
executor.