[SPARK-1520] remove fastutil from dependencies

A quick fix for https://issues.apache.org/jira/browse/SPARK-1520

By excluding fastutil, we bring the number of files in the assembly jar back under 65536, so Java 7 won't create the assembly jar in zip64 format, which cannot be read by Java 6.

With this change, the assembly jar now has about 60000 entries (58000 files), tested with both sbt and maven.