Use an int instead of a double for shuffling key

We decide the partition by calling .hashCode on the key, which
returns an Int. So generating anything larger than an Int is a
waste of space and computation (and calling .hashCode on an int is
much faster, as it merely returns the value as opposed to actually
doing any hashing)
Merge pull request #244 from NICTA/experimental-shuffle

Use an int instead of a double for shuffling key