Update sbt version to 0.13.6
Downgrade guava version to 16.0.1

Guice 4.0-beta5 has a dependency on 16.0.1 which causes newer versions
of sbt (>0.13.6) to throw warnings about incompatible libraries.
Merge pull request #40 from nlochschmidt/develop

Update sbt to 0.13.6
Fix eviction warnings when using sbt >= 0.13.6