Do not use recently added gzip option -k. Fixes #6

Since the gzip option "-k" has been added fairly recently [1], this commit
implement a workaround that (1) backup the script, (2) gzip it in place
and (3) copy the backup to its original name.

[1] http://lists.gnu.org/archive/html/bug-gzip/2013-04/msg00010.html