Automatic builds for auth-module on x86_64 (via Travis) and on-demand on-device for arm via publish.sh

The basic-auth module is not built or pushed upon 'tag' / 'release' of the faas repo, but it should be: https://github.com/openfaas/faas/tree/master/auth.  We also don't create on-device images for this, but should do for both armhf and arm64: https://github.com/openfaas/faas/blob/master/contrib/publish-arm.sh



This change addresses these challenges and also introduces a tagAndPush script to alleviate some of the recently introduced repetition in .travis.yml.

Signed-off-by: Richard Gee <richard@technologee.co.uk>