readlink error on macOS

Hello,

Nice job this script 👍 .

However [line 16](https://github.com/C-Duv/freemobile-smsapi-client/blob/master/send-notification.sh#L16) cause on my macOS this error:
```sh
$ uptime | ./sms.sh -c credentials
readlink: illegal option -- m
usage: readlink [-n] [file ...]
```