Merge branch 'stable-2.15'

* stable-2.15:
  Bump sbt to 1.2.3
  Bump Gerrit API version to 2.15.3
  Align Gerrit API version to 2.14.12
  Upgrade sbt plugins

Change-Id: If0c6d97d076ce2261ffffafd3cd0e17db89e21e8