Define dev version_map.yaml

[eslavich]

Right now the version_map file with the latest version (1.4) is functioning as the development area, but at the same time it is explicitly supported by the latest released version of the asdf library. We should define a dev version_map that we can develop on, and only release new versions of the standard when they are finalized. Otherwise users may end up with files created with the latest version that don't validate because we've modified the schemas in the meantime.