Update README.md

Author: barryvdh

README.md

@@ -11,21 +11,11 @@ processing library for PHP 5.3+. This package implements Authorize.Net support f
 
 ## Installation
 
-Omnipay is installed via [Composer](http://getcomposer.org/). To install, simply add it
-to your `composer.json` file:
-
-```json
-{
-    "require": {
-        "omnipay/authorizenet": "~2.0"
-    }
-}
-```
-
-And run composer to update your dependencies:
+Omnipay is installed via [Composer](http://getcomposer.org/). To install, simply require `league/omnipay` and `omnipay/authorizenet` with Composer:
 
-    $ curl -s http://getcomposer.org/installer | php
-    $ php composer.phar update
+```
+composer require league/omnipay omnipay/authorizenet
+```
 
 ## Basic Usage