Uodate minimum PHP version in docs.

Author: judgej

README.md

@@ -10,7 +10,7 @@
 [Omnipay](https://github.com/thephpleague/omnipay) is a framework agnostic, multi-gateway payment
 processing library for PHP 7.1+.
 
-**This is for Omnipay 3.x with a minimum PHP version of 7.1.**
+**This is for Omnipay 3.x with a minimum PHP version of 5.6.**
 
 Table of Contents
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