When building netbox-docker on Travis, it happened too often that the
build failed because of Github's API rate limits. These apply for all
requests, but if the request is unauthenticated they are applied by IP.
This is bad for netbox-docker, as we build on Travis where the build-
workers are shared with the rest of the world.
This commit adds the possibility to define OAuth credentials as
documented at [1]. Hopefully this leads to more reliable releases of
netbox-docker images.
[1] https://developer.github.com/v3/#increasing-the-unauthenticated-rate-limit-for-oauth-applications
This change introduces an API call to hub.docker.com to check if
the version we're about to build was already built & pushed. This
avoids superfluos builds.
