Standardize "curl" arguments across the repo to be or include "-sSL"

- https://botbot.me/freenode/docker-dev/msg/17315692/
- https://botbot.me/freenode/docker-dev/msg/17315994/

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6885 from tianon/standard-curl-sSL