[bug] Deploy preferene snippets before apt_refresh

When pinning packages with apt::preferences_snippet,
we need to make sure these get deployed before an
`apt-get update` is triggered, so pinned packages can
get installed in the right way with a single puppetrun.

manifests/preferences_snippet.pp

@@ -31,7 +31,8 @@ define apt::preferences_snippet (
 
   file { "/etc/apt/preferences.d/${name}":
     ensure => $ensure,
-    owner  => root, group => 0, mode => '0644';
+    owner  => root, group => 0, mode => '0644',
+    before => Exec['refresh_apt'];
   }
 
   case $source {