Bug 13377: mark automatic_renewals.pl as executable

This script is called by a cron job, but was never marked as executable,
so will error every time it's called, which is no good. This also adds
an appropriate #! line to the top of the file.

To test:
* apply the patch
* make a package
* ensure that the installed
  /usr/share/koha/bin/cronjobs/automatic_renewals.pl
  is mode 755 and does something when run.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>