Java 7 and failing unit tests

[Dave Beech]

Guys - just wanted to make you aware that some unit tests are currently 
failing in trunk when building against Java 7. I know this isn't 
strictly a problem, or even wrong since Hadoop itself requires Java 6, 
but it just caught me out for a minute because of my Eclipse setup!

Test failures:
  
testJavaSerialization(org.apache.hadoop.mrunit.mapreduce.TestMapReduceDriver): 
java.lang.Integer cannot be cast to org.apache.hadoop.io.WritableComparable
  testJavaSerialization(org.apache.hadoop.mrunit.TestMapReduceDriver): 
java.lang.Integer cannot be cast to org.apache.hadoop.io.WritableComparable
  
testJavaSerialization(org.apache.hadoop.mrunit.TestPipelineMapReduceDriver): 
java.lang.Integer cannot be cast to org.apache.hadoop.io.WritableComparable

Maybe we should add a note in BUILD.txt to say Java 6 *must* be used? It 
sort of does at the moment but I think it could be more explicit.

Dave