Add missing 3.0.0-SNAPSHOT repository to pom.xml

When trying to build this I ran into a minor problem - 3.0.0-SNAPSHOT wasn't installed, nor is it published on org.apache.hadoop. 
After running the trunk build of hadoop with mvn install, this was fixed and of course - building hadoop from scratch requires a little bit of time, and an installation of protocol buffers, etc... so its not very convenient and certainly easy to automate when snapshots change.

Solution -- 

So - after some digging it became clear that in fact apache puts all snapshotted jars on http://repository.apache.org/content/groups/snapshots-group/.  I've added that as a "repository" tag in this pull request - so that when running "mvn package", the snapshot 3.0.0 jars are taken from the snapshots-group that is maintained by apache.  

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To test this pull request : 
mv $HOME/.m2/org/apache/hadoop/hadoop-common/* /tmp/oldhadoop
mvn test 

^^ Upon running the above you will see the 3.0.0-SNAPSHOT jars redownloaded directly from repository.apache.org .