2017-05-18

Announcements:
 - Use different buffer-size values for different tile formats.
 - Official support for `buffer-size` at layergroup level.
   * Check [doc/MapConfig-1.6.0.md](doc/MapConfig-1.6.0.md) for more details.
 - Upgrade tilelive-bridge to [2.3.1-cdb2](https://github.com/CartoDB/tilelive-bridge/releases/tag/2.3.1-cdb2).
 - Upgrade tilelive-mapnik to [0.6.18-cdb2](https://github.com/CartoDB/tilelive-mapnik/releases/tag/0.6.18-cdb2).