Adding user to 'sudo' group and 'sudo' group to sudoers file does not grant sudo privileges

[Alexander Regueiro]

Steps to reproduce.

1. Create a new user, say, named `foo`.
2. Run `sudo usermod -aG sudo foo` to add the user `foo` to the `sudo` group. This can then be confirmed by inspecting the `/etc/group` file, or running `id foo`.
3. Uncomment/add the line `sudo ALL=(ALL:ALL) ALL` in the sudoers file, using `visudo`.
4. Log-in as `foo` (after rebooting, even).
5. Run `sudo echo foo`.
6. Observe the error `foo is not in the sudoers file. This incident will be reported.`.

Output of `uname -a`:
```
Linux home-media-server 4.9.41+ #1023 Tue Aug 8 15:47:12 BST 2017 armv6l GNU/Linux
```