33-aptsetup: improve perl regex to work with backports and perl 5.14
The perl command line which is responsible for replacing
ftp.debian.org with the according snapshot.debian.org entry
produced an invalid sources.list entry with perl v5.14.2,
e.g. generating:
| deb http://snapshot.debian.org/archive/debian/
20150622/ main deb-src http://snapshot.debian.org/archive/debian/
20150622/ main
with the following input:
| deb http://ftp.debian.org/debian/ jessie-backports main
| deb-src http://ftp.debian.org/debian/ jessie-backports main