temporary workaround; checkout an earlier version of fxa-auth-server

This is just offered as a workaround so the travis build of fxa-content-server works again, and a better solution is worked out. This just checks out fxa-auth-server as of last Friday for running the content server tests.

/cc @shane-tomlinson since you'll be up in an hour or so and this would matter most to you overnight(PDT).