Publish succeeds if elm-package.json is present but not included in Github

[JoeyEremondi]

Sorry to add to the elm-package issues, but I just ran into this and thought it should be logged.

Basically what the title says: if you forget to add your elm-package.json file to the GH repository, a publish will succeed without any warning. But if a user attempts to install the package, it crashes with a very un-helpful error message.

It's probably a low-priority issue, since the short-term solution is "Make sure the library maintainers don't do something stupid", but in the long run, if we're depending on Git, we should probably ensure that the repository, not just the local files, are well formed.