Jetpack tests: wait until jetpack beta build is ready

[brbrr]

Current Jetpack branch build process looks like this:

1. Travis build is triggered. If build succeeded - it triggers beta builder
2. Beta builder doing some magic and produce a Jetpack plugin archive and update list of available branches: https://betadownload.jetpack.me/jetpack-branches.json

So if build is not successful or there some glitch in Beta builder - list of available branches would not be updated.

E2E tests against master:
In this case, our tests triggered together with Travis build, which means tests will use Jetpack plugin from a previous successful merge. In most of the cases that come into my mind - this not a problem.

E2E tests against PRs:
Tests are triggered manually, so it might be (and I expect quite often) the situation when the beta branch is not ready yet.

To address this issue - we need to wait until branches list gets updated, and only after that, we should trigger CI job