Update to 1.28.x branch

The OpenStack wiki is running a MediaWiki 1.28 point release, so
upgrade to a compatible source branch in preparation for a new data
import on the dev server currently managed by this module.

Change-Id: I04ca733dd8da0dc43a90b77d9a1356888e9f0f69

manifests/app.pp

@@ -1,6 +1,6 @@
 # Class: mediawiki::app
 #
-class mediawiki::app ($revision = 'origin/REL1_27'){
+class mediawiki::app ($revision = 'origin/REL1_28'){
   vcsrepo { '/srv/mediawiki/w':
     ensure   => present,
     provider => git,

manifests/extension.pp

@@ -2,7 +2,7 @@ define mediawiki::extension (
   $type     = 'extension',
   $ensure   = latest, # keep up to date
   $source   = undef, # actual default conditionally applied below
-  $revision = 'origin/REL1_27',
+  $revision = 'origin/REL1_28',
 ) {
   if $type != 'extension' and $type != 'skin' {
     fail( '$type must be extension or skin' )