PSA: Pull requests may require a rebase

New travis jobs for branches based on something older than current 
master (i.e. branches not containing 65b840901111) may fail. This is 
because there was a change that affected both wptrunner and 
web-platform-tests, but there is no version pinning between these 
repositories.

In the short term, please accept my apologies and rebase your pull 
request if you run into difficulties.

In the longer term, this is a good reason to put all the wpt tools into 
a single repository so cross-cutting changes are easier to make in an 
atomic fashion.