[ci skip] ensure we follow redirects and use ssl when grabbing the docker gpg key

Also install curl when running `vagrant up dokku-deb`

Makefile

@@ -86,7 +86,7 @@ docker: aufs
 	apt-get install -qq -y curl
 	egrep -i "^docker" /etc/group || groupadd docker
 	usermod -aG docker dokku
-	curl --silent https://get.docker.com/gpg | apt-key add -
+	curl -sSL https://get.docker.com/gpg | apt-key add -
 	echo deb http://get.docker.io/ubuntu docker main > /etc/apt/sources.list.d/docker.list
 	apt-get update
 ifdef DOCKER_VERSION