=pro Grab binary version for MIMA combatibility testing from a setting.

Additionally, also use Akka 2.3.2 for checking Scala 2.11 binary
compatibility as there was no Akka 2.3.0.
Merge pull request #16137 from jsuereth/wip/mima-fixins

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