Bump to 2.5.1

Summary: as above

Test Plan: NP

Author: Luca Bruno

src/FacebookAds/Api.php

@@ -35,7 +35,7 @@ class Api {
   /**
    * @var string
    */
-  const VERSION = '2.5.0';
+  const VERSION = '2.5.1';
 
   /**
    * @var Api