Fix the class cast exception in EtlMultiOutputRecordWriter

We saw the following exception when we received very old kafka message:

camus-hourly-Lix INFO - task error: Error: java.lang.ClassCastException: org.apache.hadoop.mapred.TaskAttemptContextImpl cannot be cast to org.apache.hadoop.mapreduce.Mapper$Context
18-12-2014 14:16:08 PST camus-hourly-Lix INFO - 	at com.linkedin.camus.etl.kafka.mapred.EtlMultiOutputRecordWriter.write(EtlMultiOutputRecordWriter.java:87)

Actually there is no need to cast TaskAttemptContextImpl to Mapper$Context since TaskAttemptContextImpl have the getCounter method already.