update changelog for 2.15.1 release
### Development
-Bug Fixes:
+### 2.15.1 / 2018-09-01
+
- Prevent crash caused by internal Android exception when stopping scanning (#724, David G. Young)
- Fix Android 8 crashing apps on background monitoring/ranging data delivery (#720, David G. Young)
- Fix intermittent NPE on ranging beacons (#716, Federico Bertoli, David G. Young)
