some tweaks to .travis.yml

- use github emacs mirror for travis test: repo.or.cz was down a few
  times; since github.com must be up to run anything anyway, it's
  preferable to rely on that instead.

- run all the apt commands separately; there have been some intermittent
  failures, this should make it easier to diagnose.

- suppress curl progress bar

- show emacs --version before running tests