Fix HDFS access bug with assembly build.

Due to this change in HDFS:
https://issues.apache.org/jira/browse/HADOOP-7549

there is a bug when using the new assembly builds. The symptom is that any HDFS access
results in an exception saying "No filesystem for scheme 'hdfs'". This adds a merge
strategy in the assembly build which fixes the problem.
Merge pull request #921 from pwendell/master

Fix HDFS access bug with assembly build.