We're passing a shell conditional to `deploy.script`, however Travis CI
seems to get confused about this in the deploy section, causing
GNU/Linux to run `make publish-aws-s3` directly on the Travis CI build
instead of in the Docker container, which causes the deployment to
eventually fail because of missing dependencies.
According to Travis CI:
> `deploy.script` must be a scalar pointing to an executable file or
> command.
See https://travis-ci.org/resin-io/etcher/jobs/214723725 for a failure
example.
We also move the Appveyor deploy script to a separate file for symmetry
purposes.
Signed-off-by: Juan Cruz Viotti <jviotti@openmailbox.org>
