check_packages should find security updates on the official security mirror too

[1] says one should use the following line to get security updates for Debian:
  deb http://security.debian.org/ wheezy/updates main contrib non-free
this produces the following `apt-cache policy <pkg>` output:
  500 http://security.debian.org/ wheezy/updates/main amd64 Packages
which does not match the current regex, as this looks for `debian-security`
(which is only present on "regular" Debian mirrors).

The new regex matches security.debian.org and also the (obsolete)
security.eu.debian.org and security.us.debian.org names.

[1] http://www.debian.org/security/
Merge pull request #15 from evgeni/master

check_packages should find security updates on the official security mirror too