Fix confusing kerl usage string for `update`

[lwalkin@lwalkin:~]> ./kerl update
usage: ./kerl  <releases>

should be:

[lwalkin@lwalkin:~]> ./kerl update
usage: ./kerl update releases

This patch fixes that.
Update kerl
Merge pull request #73 from vlm/patch-1

Fix confusing kerl usage string for `update`