Authored by Justin R. Miller

update changelog for 1.6.0

Changelog
---------
### 1.6.0
#### February 4, 2015
- Removed support for Mapbox `v3` API and require [access tokens](https://www.mapbox.com/developers/api/#access-tokens).
- Fixed a small bug with database caching which was also causing an error in the console log.
- Reduced console log verbosity.
- Deprecated the `-[RMMapboxSource init]` convenience initializer since tokens are now required.
- Updated the included GRMustache from 6.8.3 to 7.3.0.
- Documentation improvements.
### 1.5.1
#### January 28, 2015
... ...