ensure Request::env('REMOTE_ADDR') consistently returns only one IP address

In some environments, such as Orchestra.io's load-balanced nginx environment, `$_SERVER['HTTP_X_FORWARDED_FOR']` will actually contain three IP addresses concatenated by comma+space.

When running `Request::env('REMOTE_ADDR')`, I'd like to reasonably expect that I'm only going to get the remote IP address of the client.

see [stackoverflow.com](http://stackoverflow.com/questions/753645/how-do-i-get-the-correct-ip-from-http-x-forwarded-for-if-it-contains-multiple-ip) and [wikipedia.org](http://en.wikipedia.org/wiki/X-Forwarded-For)