add_repo.sh uses http to download public key

```
wget -O - http://dl.concerto-signage.org/concerto_deb_public.key | sudo apt-key add -
```

This download uses plain HTTP, thus the authenticity of the public key is not guaranteed. It should be:
```
wget -O - https://dl.concerto-signage.org/concerto_deb_public.key | sudo apt-key add -
```