Updated requirements (KnpLabs#689)

Author: Nyholm

README.md

@@ -20,7 +20,8 @@ Uses [GitHub API v3](http://developer.github.com/v3/) & supports [GitHub API v4]
 ## Requirements
 
 * PHP >= 5.6
-* [Guzzle](https://github.com/guzzle/guzzle) library,
+* A [HTTP client](https://packagist.org/providers/php-http/client-implementation)
+* A [PSR-7 implementation](https://packagist.org/providers/psr/http-message-implementation)
 * (optional) PHPUnit to run tests.
 
 ## Install