update failover for DW to use fastly

Most of our services will still failover using DNS, but we're experimenting with fastly for the DW.

There's a fastly rubygem, but the API is a little bit clunky. The configuration is versioned, so you make changes to a draft version and then activate it. It ends up being a bit verbose, so I wrapped it in a class that makes some assumptions about how we use fastly and presents a simple API.

Depending on how this goes, we might expand to use fastly for other apps.