Enable cross-build

According to
[Cross-Build](http://www.scala-sbt.org/0.12.1/docs/Detailed-Topics/Cross-Build.html) it
is possible to make sbt build through a set of target scala versions and
according to
[Library-Deps](http://www.scala-sbt.org/0.12.1/docs/Getting-Started/Library-Dependencies.html#getting-the-right-scala-version-with)
the current Scala version is use can be applied to a dependency.

I ran

```
sbt clean; sbt +test;
```

and it was successful. The plus sign is there to enable the cross-build.