explicitly checking for simple pages in ACL

This was causing a problem when upgrading Omeka, since the ACL role didn't exist while the plugin needed an upgrade, but it was still passing the 'is active' check

CourseToolsPlugin.php

@@ -53,7 +53,7 @@ public function hookDefineAcl($args)
     $acl->allow('student', 'Elements', 'element-form');
 
     # Set student permissions on Simple Pages
-    if (plugin_is_active("SimplePages")) {
+    if ($acl->has('SimplePages_Index') && $acl->has('SimplePages_Page')) {
       if (get_option('simple-page-access')) {
         $acl->allow('student', array('SimplePages_Index', 'SimplePages_Page'));
       }