Target java 1.7

I think the artifact you're publishing is still java 1.8. I tweaked the build settings and confirmed them using javap.

```
$ javap -verbose -classpath target/scala-2.10/akka-persistence-kafka_2.10-0.4-SNAPSHOT.jar  akka/persistence/kafka/snapshot/SnapshotMetadataFormat$SnapshotMetadata | grep major
  major version: 51
```