README: Switch to CircleCI build status badge

We don't use TravisCI for regular builds anymore.

README.md

@@ -1,8 +1,6 @@
-Helios
+Helios [![Circle CI](https://circleci.com/gh/spotify/helios.png?style=badge)](https://circleci.com/gh/spotify/helios)
 ======
 
-[![Build Status](https://travis-ci.org/spotify/helios.svg?branch=master)](https://travis-ci.org/spotify/helios)
-
 Helios is a Docker orchestration platform for deploying and managing
 containers across an entire fleet.
 
@@ -231,4 +229,4 @@ order):
 * Run once jobs -- for batch jobs
 * Resource specification and enforcement -- That is: restrict my container to *X* MB of RAM, *X* CPUs, and *X* MB disk and perhaps other things like IOPs, network bandwidth, etc.
 * Dynamic scheduling of jobs -- either within Helios itself or as a layer on top
-* Packaging/Config for other Linux OS's like RedHat, CoreOS, etc.
+* Packaging/Config for other Linux OS's like RedHat, CoreOS, etc.