upgrade org.iq80.leveldb to 0.7

Upgrade org.iq80.leveldb to the latest version 0.7 because the version 0.5 does not play nicely in a project using Guava >= 16. 

```
[ERROR] Uncaught error from thread [omnibus-akka.persistence.dispatchers.default-plugin-dispatcher-9] shutting down JVM since 'akka.jvm-exit-on-fatal-error' is enabled for ActorSystem[omnibus]
[ERROR] java.lang.NoSuchMethodError: com.google.common.io.Closeables.closeQuietly(Ljava/io/Closeable;)V
[ERROR]  at org.iq80.leveldb.impl.MMapLogWriter.close(MMapLogWriter.java:83)
[ERROR]  at org.iq80.leveldb.impl.VersionSet.initializeIfNeeded(VersionSet.java:111)
[ERROR]  at org.iq80.leveldb.impl.VersionSet.<init>(VersionSet.java:91)
[ERROR]  at org.iq80.leveldb.impl.DbImpl.<init>(DbImpl.java:167)
[ERROR]  at org.iq80.leveldb.impl.Iq80DBFactory.open(Iq80DBFactory.java:59)
[ERROR]  at akka.persistence.journal.leveldb.LeveldbStore$class.preStart(LeveldbStore.scala:114)
[ERROR]  at akka.persistence.journal.leveldb.LeveldbJournal.preStart(LeveldbJournal.scala:19)
[ERROR]  at akka.actor.Actor$class.aroundPreStart(Actor.scala:470)
[ERROR]  at akka.persistence.journal.leveldb.LeveldbJournal.aroundPreStart(LeveldbJournal.scala:19)
[ERROR]  at akka.actor.ActorCell.create(ActorCell.scala:580)
[ERROR]  at akka.actor.ActorCell.invokeAll$1(ActorCell.scala:456)
[ERROR]  at akka.actor.ActorCell.systemInvoke(ActorCell.scala:478)
[ERROR]  at akka.dispatch.Mailbox.processAllSystemMessages(Mailbox.scala:263)
[ERROR]  at akka.dispatch.Mailbox.run(Mailbox.scala:219)
[ERROR]  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
[ERROR]  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
[ERROR]  at java.lang.Thread.run(Thread.java:724)
```

Leveldb 0.5 uses the API com.google.common.io.Closeables closeQuietly(Closeable closeable) that has been removed in Guava 16.

Version 0.7 fixes this problem and uses Guava 16.0.1.