Bug in getNotifications methos

[diogocoutinho]

The getNotifications with a bug of write.

Error:

In the line 430 of OneSignalClient:

        if($offset) {
            $endpoint.="&offset=".$$offset;
        }

Here seems a double "$" and caused a error when provide a offset by user

Solution:

Just remove a "$"

        if($offset) {
            $endpoint.="&offset=".$offset;
        }

I will a PR to help.