Added javac source and target versions

To ensure that a compatible with JVM 7 bytecode is produced.
Current version of the library is built against Java 8:
http://search.maven.org/remotecontent?filepath=com/github/michaelpisula/akka-persistence-inmemory_2.10/0.1.1/akka-persistence-inmemory_2.10-0.1.1.jar

>  javap -verbose akka.persistence.journal.inmemory.InMemoryJournal | grep "major"
  major version: 52

Which cause errors on JVM 7:

java.lang.UnsupportedClassVersionError: akka/persistence/journal/inmemory/InMemoryJournal : Unsupported major.minor version 52.0

It would be great to use akka-journal-inmemory with JVM 7 as Java 8 is not widely used yet and I believe most of the production Scala-Akka code is ran against JVM 7.