Change build badge to point to Circle CI

Author: davidgyoung

README.md

@@ -1,7 +1,7 @@
 Android Beacon Library
 =======================
 
-[![Build Status](https://snap-ci.com/AltBeacon/android-beacon-library/branch/master/build_image)](https://snap-ci.com/AltBeacon/android-beacon-library/branch/master)
+[![Build Status](https://circleci.com/gh/AltBeacon/android-beacon-library.png?circle-token=4e11fb0dccaa8b98bc67fdbe38b179e4a7d07c27)](https://circleci.com/gh/AltBeacon/android-beacon-library)
 
 An Android library providing APIs to interact with beacons.  Please visit the
 [project website](http://altbeacon.github.io/android-beacon-library/) for how to use this library.