handle https:// key_sources

discovered on ubuntu trusty with gnupg 1.4.16-1ubuntu2.1:

gpg --fetch-keys https://download.newrelic.com/548C16BF.gpg
gpgkeys: protocol `https' not supported
gpg: no handler for keyserver scheme `https'
gpg: WARNING: unable to fetch URI https://download.newrelic.com/548C16BF.gpg: keyserver error