Add goreleaser setup

[pcarranza]

Changes:

• Add a couple of other dockerfiles for armv6 and arm64
• Add a goreleaser configuration file with some assumptions that may need some reviewing
• Simplify makefile as it's now using goreleaser to control builds

To build now just make, to release make release

[pcarranza]

Humm.... seems like this is a limitation in Travis, I wonder if it makes sense to just build in travis but not generate the docker images.