Prod server: force ipv4 in apt

Without this, apt seems to try to use ipv6 to connect to some of the
repos, and it often hangs or fails.

salt/salt/prod-config.sls

@@ -0,0 +1,4 @@
+# Force apt not to use IPv6 (seems to hang often)
+/etc/apt/apt.conf.d/99force-ipv4:
+  file.managed:
+    - contents: 'Acquire::ForceIPv4 "true";'

salt/salt/top.sls

@@ -30,6 +30,7 @@ base:
   'prod':
     - nginx.static-sites-config
     - raven
+    - prod-config
   'monitoring':
     - nginx
     - self-signed-cert