Fix type error in graphite server.

When you start a graphite server, there's a type error:

```
java.lang.ClassCastException: riemann.transport.graphite$graphite_frame_decoder$fn__7763 cannot be cast to org.jboss.netty.channel.ChannelHandler
	at riemann.transport.graphite$graphite_server$reify__7776.getPipeline(graphite.clj:66)
	at org.jboss.netty.channel.socket.nio.NioServerBoss.registerAcceptedChannel(NioServerBoss.java:134)
	at org.jboss.netty.channel.socket.nio.NioServerBoss.process(NioServerBoss.java:104)
	at org.jboss.netty.channel.socket.nio.AbstractNioSelector.run(AbstractNioSelector.java:313)
	at org.jboss.netty.channel.socket.nio.NioServerBoss.run(NioServerBoss.java:42)
	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
	at java.lang.Thread.run(Thread.java:680)
```

This patch fixes it.