Incorrect date format in Release file

[mpreissner]

Using the script to sign an Apt repo as-is generates a Date field consisted with the configured timezone on the server. On clients (tested Ubuntu 16.04), this results in a "Invalid 'Date' entry in Release file". Change the DATE variable in the script to use "DATE=date -R -u produces a timestamp in the correct format for deb repositories and fixes the error when running apt-get update on clients.