Fix gnupg is required by apt-get update on Debian

Open username-removed-1628446 requested to merge hugoherter/www-gitlab-com:patch-440 into master

On a fresh minimal install of Debian 9/Stretch, apt-get update was not recognizing the signature of the GitLab repository due to gnupg not present by default.

This fixes the issue on Debian by adding gnupg to the packages to install before running the installation script.