Use GitHub actions instead of Travis

[bastilimbach]

See #25 (comment)

[Legion2]

@bastilimbach What is this line for?

docker-MagicMirror/build.sh

Line 6 in 0d51263

if [ "$(docker manifest inspect bastilimbach/docker-magicmirror:"${latest_release}" > /dev/null; echo $?)" != 0 ]; then

[bastilimbach]

@Legion2 it checks if there is already an image with that particular tag/version on docker hub. This is to prevent overriding existing tags.

[Legion2]

But there should be no problem if we override a tag with a newer build of the same tag. It uses the same code base but maybe newer base images and dependencies.