Docker hub TOS changes

[t-b]

See 1 for their updated terms of services.

I don't think we need to worry about inactive-images-will-be-deleted.

More interesting is the changes in how often you can pull images.

for anonymous users: 100 pulls per 6 hours per IP address

We are pulling two images for each travis job with running tests enabled (five, I'm already counting the static library job from #758.). So this means 5 pulls per image per PR. And that means 20 PRs in six hours. So this should be good for now. If not we should move the images to github.

[t-b]

Nothing to do for now.