apt.sh: fix quoting

Apparently users don't want to double quote arguments ...

Thanks to Uli for reporting this.

Signed-off-by: vsp <vsp@chuck.Informatik.Uni-Oldenburg.DE>
lib/checkstyle: update from version 5.6 to 6.1.1

Signed-off-by: vsp <vsp@chuck.Informatik.Uni-Oldenburg.DE>