Case sensitivity

[TonyGrima]

Thanks for the package - works fine for me but I found a problem in Instagram.php at line 633. This causes a failure but if I change 'X-Ratelimit-Remaining' to lower case it works.

 $this->_xRateLimitRemaining = $headers['X-Ratelimit-Remaining'];

to:
$this->_xRateLimitRemaining = $headers['x-ratelimit-remaining'];