Disabling Travis?

[Boespflug, Mathieu]

It took me no more than a couple hours to get this working, but using
CircleCI, for our fork of GHC. I started from Joachim's TravisCI
script.

https://circleci.com/gh/tweag/ghc/tree/circleci

It would be trivial to activate this for github.com/ghc/ghc as well.

A few notes:
- It runs ./validate --fast in 40 minutes.
- CircleCI has OS X support as well. I think we should just migrate to
using CircleCI for OS X testing instead of the custom drones, one or
all of which are currently down.
- CircleCI graciously agreed to running on one of the beefy AWS node
types, called c4.xlarge (8 cores). On the standard node type (2
cores), validate takes just over an hour to run. It would be great if
./validate could scale better to more cores.