refactor: took changes from dev and put them in master

src/QUI/Memberships/Utils.php

@@ -85,7 +85,8 @@ public static function getInstalledMembershipPackages()
         $packages         = array();
         $relevantPackages = array(
             'quiqqer/products',
-            'quiqqer/contracts'
+            'quiqqer/contracts',
+            'quiqqer/erp-plans'
         );
 
         foreach ($relevantPackages as $package) {
@@ -110,6 +111,16 @@ public static function isQuiqqerProductsInstalled()
         return QUI::getPackageManager()->isInstalled('quiqqer/products');
     }
 
+    /**
+     * Check if quiqqer/erp-plans is installed
+     *
+     * @return bool
+     */
+    public static function isQuiqqerErpPlansInstalled()
+    {
+        return QUI::getPackageManager()->isInstalled('quiqqer/erp-plans');
+    }
+
     /**
      * Check if quiqqer/contracts is installed
      *