Pull latest tag by default

This was changed in Docker recently:

https://github.com/docker/docker/pull/7759

This means we aren't pulling loads of tags when we only use the
latest.

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #716 from bfirsh/pull-latest-tag-by-default

Pull latest tag by default