Fix MantisCore minimum version

The required version is actually 2.1 and not 2.5. Also, take this
opportunity to move the project to semantic versioning as the
other plugins developed by Intra2net.

Author: samiraguiar

ProjectNotifications/ProjectNotifications.php

@@ -22,9 +22,9 @@ public function register() {
         $this->description = plugin_lang_get( "description" );
         $this->page = '';
 
-        $this->version = "1.0";
+        $this->version = "1.0.0";
         $this->requires = array(
-            "MantisCore" => "2.5.0",
+            "MantisCore" => "2.1.0",
         );
 
         $this->author = "Intra2net AG";