Added sbt plugin repo to fix importing into intellij

[peteanning]

Needed the sbt plugin repo to import directly into Intellij as an sbt based project

[lightbend-cla-validator]

Hi @peteanning,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Lightbend Contributors License Agreement:

http://www.lightbend.com/contribute/cla

[wsargent]

You should not need this -- IntelliJ can install projects directly from build.sbt.