Remove genisoimage from dependencies

Remove genisoimage from dependencies

When running grml-live with genisoimage (9:1.1.11-3+b2) on Debian/stretch the
following error is shown:

  # ./grml-live -s sid -a amd64 -c GRMLBASE,GRML_SMALL,AMD64 -t $(pwd)/templates/ -o /dev/shm/grml-live
  [...]
    [*] Finished execution of stage 'squashfs'
    [*] Forcing rebuild of ISO because files on ISO have been modified.
    [*] Using genisoimage to build ISO.
  genisoimage: -i option no longer supported.
  stat: cannot stat '/dev/shm/grml-live/grml_isos/grml_0.0.1.iso': No such file or directory
    [!] Error: there was a critical error executing stage 'iso build

Closes: grml/grml-live#65