Build edxapp image and push to docker hub from Travis

[thraxil]

This should push the edxapp docker image to appsembler/edxapp:hawthorn after a successful merge to appsembler/hawthorn/master.

The docker hub credentials are in TravisCI's settings for the repo. I tested this out by doing the build/push step in the main script section and it seems to work fine.

The only real thing to be aware of with this is that it takes a good 25 minutes to build and push the image. So people will need to understand that they can't docker pull immediately after a PR is merged and expect to get the new image. But this should still be better than requiring one of us to do it manually every time.

Some questions for discussion:

• is the hawthorn tag the only one we care about? should we also make sure we push to latest? (devstack uses hawthorn, right?)
• we could pretty easily have it also push an image for each PR tagged with the branch name or SHA1. I left that out for now but is that something anyone else wants? It just adds a long delay to the time Travis takes to run the tests and report success back to the PR.

[thraxil]

OK. Switched it to latest. The only problem with that is that we will have conflicts once we are also dealing Ironwood, etc.

[OmarIthawi]

@thraxil either lastest or hawthorn.master works fine, but let's worry now about Hawthorn. Fixing that minor issue for Ironwood is a tax that I'd be happy to pay when that day comes -- the day when we want to migrate form Hawthorn to Ironwood.