Update scala, sbt and dependencies versions

[beatriz]

I've upgraded the scala and sbt versions as well as the dependencies so that I can use it in a project. Hope it's ok and still releasable 😃

[beatriz]

The test that calls http://betty.userland.com/RPC2 is failing due to the response having the Content-Type as text/plain. Not sure if the header should be set before unmarshalling (which seems kind of weird) or if the test should be changed altogether.