Bump up compatibility

Author: steveamaza

README.txt

@@ -3,7 +3,7 @@ Contributors: paystack, kendysond, steveamaza
 Donate link: https://paystack.com/demo
 Tags: paystack, recurrent payments, nigeria, mastercard, visa, target, Naira, payments, verve, donation, church, NGO, form, contact form 7, form
 Requires at least: 3.1
-Tested up to: 5.0
+Tested up to: 5.1
 Stable tag: 3.3.2
 License: GPLv2 or later
 License URI: http://www.gnu.org/licenses/gpl-2.0.html