Update Omnipay to reflect The PHP League link.

README.md

 ## E-commerce
 *Libraries and applications for taking payments and building online e-commerce stores.*
 
-* [OmniPay](https://github.com/adrianmacneil/omnipay) - A framework agnostic multi-gateway payment processing library.
+* [OmniPay](https://github.com/thephpleague/omnipay) - A framework agnostic multi-gateway payment processing library.
 * [Payum](https://github.com/payum/payum) - A payment abstraction library.
 * [Sylius](http://www.sylius.org/) - An open source e-commerce solution.
 * [Thelia](http://thelia.net/v2/) - Another open source e-commerce solution.