java.lang.ClassCastException: java.lang.String cannot be cast to org.apache.avro.util.Utf8

AVRO schemas can be created with different string types, ie. 'UTF8' and 'String'.
Using "avro.java.string": "String" resulted in the following exception:

```
Error: cascading.tuple.TupleException: unable to read from input identifier: hdfs://ns1/user/perplexa/log/hourly/2014/08/30/12/log.0.11.989586.7779487783.1409400000000.avro
	at cascading.tuple.TupleEntrySchemeIterator.hasNext(TupleEntrySchemeIterator.java:127)
	at cascading.flow.stream.SourceStage.map(SourceStage.java:76)
	at cascading.flow.stream.SourceStage.run(SourceStage.java:58)
	at cascading.flow.hadoop.FlowMapper.run(FlowMapper.java:127)
	at org.apache.hadoop.mapred.MapTask.runOldMapper(MapTask.java:430)
	at org.apache.hadoop.mapred.MapTask.run(MapTask.java:342)
	at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:167)
	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:1554)
	at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:162)
Caused by: java.lang.ClassCastException: java.lang.String cannot be cast to org.apache.avro.util.Utf8
	at cascading.avro.AvroToCascading.fromAvroMap(AvroToCascading.java:109)
	at cascading.avro.AvroToCascading.fromAvro(AvroToCascading.java:83)
	at cascading.avro.AvroToCascading.fromAvroUnion(AvroToCascading.java:140)
	at cascading.avro.AvroToCascading.fromAvro(AvroToCascading.java:62)
	at cascading.avro.AvroToCascading.parseRecord(AvroToCascading.java:49)
	at cascading.avro.AvroScheme.source(AvroScheme.java:254)
	at cascading.tuple.TupleEntrySchemeIterator.getNext(TupleEntrySchemeIterator.java:140)
	at cascading.tuple.TupleEntrySchemeIterator.hasNext(TupleEntrySchemeIterator.java:120)
	... 10 more
```

This patch fixed the issue and makes the code work with both, UTF8 and String types.