Update lib/Buzz/Client/AbstractCurl.php

Improved curl proper installation/initialization check

Without this check, if you have curl not installed/misconfigured, the `RuntimeException` is never thrown because everything is stopped by the following error:

    Fatal error: Call to undefined function Buzz\Client\curl_init() in .../vendor/kriswallsmith/buzz/lib/Buzz/Client/AbstractCurl.php on line 26