Add php-mbstring and php-xml distro packages

The system packages for php-mbstring and php-xml are required by
MediaWiki, so add them. While we're here, alphabetize the list.

Change-Id: I61d7af1dd4906ae724c2fe850290714e3b092a08

manifests/php.pp

@@ -1,13 +1,17 @@
 # Class: mediawiki::php
 #
 class mediawiki::php {
-  package { ['php',
-    'php-cli',
-    'php-mysql',
-    'php-apcu',
-    'php-intl',
-    'php-openid',
-    'php-memcached']:
+  package { [
+      'php',
+      'php-apcu',
+      'php-cli',
+      'php-intl',
+      'php-mbstring',
+      'php-memcached',
+      'php-mysql',
+      'php-openid',
+      'php-xml',
+    ]:
     ensure => present,
   }
   # TODO: apc configuration