docker build changed to use openjdk

[Stephen Mallette <sp...@gmail.com>]

Travis builds have been failing gloriously for a couple of weeks now. The
problem was related to docker and more specifically to our docker image
that utilized the Oracle JDK which now has this issue with its ppa (i.e.
discontinued):

https://launchpad.net/~webupd8team/+archive/ubuntu/java

I switched to Open JDK and both docker and travis are happy again. Please
note that you will need to kill your docker images to get the update. I'm
going to go through and rebase the failed PRs so that stuff looks all green
again. Probably won't bug folks who submitted external PRs. Anyway, should
be looking nice soon enough.