Change InMemory to use maven artifact

The InMemory journal is now available via maven, can we replace the dependency entry with the one shown below:

* "com.github.michaelpisula" %% "akka-persistence-inmemory" % "0.1.2"

-Todd
Merge pull request #10 from tsindot/patch-1

Upgrade akka-persistence-inmemory to 0.1.2