Upgrade to SBT v0.13.1
Keep cross-compilation just so we don't forget how it's done...

...but as the library no longer supports 2.9, there's not much point in
it at the moment. All 2.10.x versions are supposed to be binary compatible,
so no point in compiling for more than one of those.
sbt-release & sbt-sonatype
Fix url!
Update deprecated scalatest import
Merge pull request #30 from guardian/maven-central

Maven central