fix the name of ratelimit header returned from API

x-ratelimit-remaining instead of X-Ratelimit-Remaining

src/Instagram.php

@@ -630,7 +630,7 @@ protected function _makeCall($function, $auth = false, $params = null, $method =
         $headers = $this->processHeaders($headerContent);
 
         // get the 'X-Ratelimit-Remaining' header value
-        $this->_xRateLimitRemaining = $headers['X-Ratelimit-Remaining'];
+        $this->_xRateLimitRemaining = $headers['x-ratelimit-remaining'];
 
         if (!$jsonData) {
             throw new InstagramException('Error: _makeCall() - cURL error: ' . curl_error($ch));