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