update_modprobe_conf.sh don't try to read a non-existent file

Check for the presence of /etc/modprobe.d/modprobe.conf before trying to
read it
Merge remote-tracking branch 'origin/ubuntu-12.04' into ubuntu-13.04
Merge remote-tracking branch 'origin/ubuntu-13.04' into ubuntu-13.10
Merge remote-tracking branch 'origin/ubuntu-13.10' into fedora-20
Merge remote-tracking branch 'origin/fedora-20'