Force version of sfl4j to 1.6 to ensure byte compatability in production

/cc @markhibberd @raronson 

```
2014-07-10 05:49:49,436 FATAL [main] org.apache.hadoop.mapreduce.v2.app.MRAppMaster: Error starting MRAppMaster
java.lang.NoSuchMethodError: org.slf4j.helpers.MessageFormatter.format(Ljava/lang/String;Ljava/lang/Object;Ljava/lang/Object;)Lorg/slf4j/helpers/FormattingTuple;
	at org.slf4j.impl.Log4jLoggerAdapter.info(Log4jLoggerAdapter.java:345)
	at amazon.emr.metrics.MetricsSaver.<init>(MetricsSaver.java:202)
	at amazon.emr.metrics.MetricsSaver.ensureSingleton(MetricsSaver.java:403)
	at amazon.emr.metrics.MetricsSaver.addInternal(MetricsSaver.java:228)
	at amazon.emr.metrics.MetricsSaver.addValueWithError(MetricsSaver.java:163)
	at org.apache.hadoop.ipc.ProtobufRpcEngine$Invoker.invoke(ProtobufRpcEngine.java:211)
	at com.sun.proxy.$Proxy9.getFileInfo(Unknown Source)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.apache.hadoop.io.retry.RetryInvocationHandler.invokeMethod(RetryInvocationHandler.java:186)
	at org.apache.hadoop.io.retry.RetryInvocationHandler.invoke(RetryInvocationHandler.java:102)
	at com.sun.proxy.$Proxy9.getFileInfo(Unknown Source)
	at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolTranslatorPB.getFileInfo(ClientNamenodeProtocolTranslatorPB.java:651)
	at org.apache.hadoop.hdfs.DFSClient.getFileInfo(DFSClient.java:1679)
	at org.apache.hadoop.hdfs.DistributedFileSystem$17.doCall(DistributedFileSystem.java:1106)
	at org.apache.hadoop.hdfs.DistributedFileSystem$17.doCall(DistributedFileSystem.java:1102)
	at org.apache.hadoop.fs.FileSystemLinkResolver.resolve(FileSystemLinkResolver.java:81)
	at org.apache.hadoop.hdfs.DistributedFileSystem.getFileStatus(DistributedFileSystem.java:1102)
	at org.apache.hadoop.fs.FileSystem.exists(FileSystem.java:1399)
	at org.apache.hadoop.mapreduce.v2.app.MRAppMaster.serviceInit(MRAppMaster.java:280)
	at org.apache.hadoop.service.AbstractService.init(AbstractService.java:163)
	at org.apache.hadoop.mapreduce.v2.app.MRAppMaster$1.run(MRAppMaster.java:1445)
	at java.security.AccessController.doPrivileged(Native Method)
	at javax.security.auth.Subject.doAs(Subject.java:415)
	at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1491)
	at org.apache.hadoop.mapreduce.v2.app.MRAppMaster.initAndStartAppMaster(MRAppMaster.java:1442)
	at org.apache.hadoop.mapreduce.v2.app.MRAppMaster.main(MRAppMaster.java:1375)
```
Another alternative might be to exclude slf4j completely and pick one up from Hadoop. I haven't tried that, but happy to investigate if we think it's a better idea.