add readiness probe to ensure deployment is ready before testing

[minrk]

We should add a readiness probe that verifies that the deployment is responding to requests on the public URLs (i.e. that the servers are up and kube-lego has done its business). This ought to prevent failed deploys on Travis that occur when the tests start faster than the deployment becomes ready.

I put a band-aid on #180 in the mean-time, but that should go away when we get a proper check.

[choldgraf]

I believe we now have this in the deploy travis script, so closing this! https://github.com/jupyterhub/mybinder.org-deploy/blob/master/.travis.yml#L56