Using $RANDOM instead of date.

I used to get random values with $(date '+%N') which prints a number of
nanoseconds. It was random enough for my purpose but still an ugly hack.
Thanks to Mitsukarenai [1], I learned about $RANDOM which I found to be
much more explicit.

[1] http://lehollandaisvolant.net/?mode=links&id=20131216124411