Docker "MAINTAINER" instruction is deprecated

[michaelherold]

Docker deprecated the "MAINTAINER " instruction in favor of "LABEL maintainer ". Perhaps we should update the CodeClimate Engine Spec to reflect this change in thinking?

See: https://docs.docker.com/engine/reference/builder/#/maintainer-deprecated

[dblandin]

Definitely! I'll see about making that spec change 👌

[dblandin]

Updated in #53. Thanks for the suggestion!