Fixed implicit conversion error for Duration comparison.

At line 65 in ```Cluster.scala``` the comparison of ```Duration``` objects does an implicit conversion to ```scala.Long```.

In Cassie 0.20.0:
```
java.lang.NoSuchMethodError: com.twitter.conversions.time$.timelikeToLong(Lcom/twitter/util/TimeLike;)J
at com.twitter.cassie.Cluster.keyspace(Cluster.scala:65)
```

Using the compare method from ```Duration``` is an easy fix.

As per the official docs for version 6.x+:
The (dangerous) implicit conversion from a Duration to a Long was removed.