Modernize SBT Dependency Versions

* SBT 0.13.7 => 0.13.7 - Faster dependency resolution
* Add -feature compile option to get feature warnings
* sbt-scaliariform 1.2.1 => 1.3.0
* sbt-git 0.6.2 => 0.6.4
* sbt-unidoc 0.3.0 => 0.3.1
* Fix deprecation warning in ReactiveMongo.scala: Defaults.defaultSettings => Defaults.coreDefaultSettings