Install poedit from wheezy-backports (Tails#6380).

We've uploaded the version we want there, no need to fetch the package from
sid anymore.

config/chroot_apt/preferences

@@ -15,8 +15,9 @@ Package: florence
 Pin: release o=Debian Backports,n=wheezy-backports
 Pin-Priority: 999
 
+Explanation: override deb.tails.boum.org's Squeeze -specific package.
 Package: poedit
-Pin: release o=Debian,a=unstable
+Pin: release o=Debian Backports,n=wheezy-backports
 Pin-Priority: 1010
 
 Explanation: override deb.tails.boum.org's Squeeze -specific package.