Fix the definition of akkaPreviousArtifact so it uses the build's scalaVersion correctly

This should enable artifacts that don't use binary cross-versioning to work, additionally this will let mima work in the presence of `++2.11.x` bumps.

I'm not sure on your methodoology of where you patch things.  These are aimed at the release-2.3 branch because that's where I'm seeing the issue.  I'm also seeing binary incompatibilities in your 2.3 branch with or without this patch, just so you're aware.

Let me know where these commits should be targeted.

Reivew by @2m cc @rkuhn 
In your build, `akkaPreviousArtifact` is never called with arguments.  I can remove all the parameters.

Also, regarding pattern match vs. if statement, I prefer the if-statement here as it denotes the openness of the match *AND* it would allow you to support more than one scala version, if such a thing happens (e.g. if you use compiler plugins you'd need to match on 2.11.1 vs just 2.11).