Add test to ensure output submodule is up to date.

[itsjohncs]

Jordan ran into an error when publishing this morning because the output submodule wasn't updated after my last change. To prevent this in the future I should add a test that only runs when testing the gh-pages branch (which I hope I can do...) that will check where the master branches output submodule points to.

[itsjohncs]

It's also possible for the output submodule to be pointing too far ahead. Meaning the user hasn't pushed the output changes. This should also be checked for.

I think some more tooling in the makefile could help with this, as well as some docs to explain what's going on. Alternatively, it might make sense to drop the submodule stuff and do an alternative build process. That potentially won't be much easier though since we'll have to clone a separate repo or do some weird swapping stuff...

[itsjohncs]

I tweaked the deploy process to automate things a bit more, and make it clearer when things fail (like they did last night which caused this nonsense).

[sophiebits]

It's possible to have Travis run on master, build the site, and push automatically to gh-pages. That's what we do on React.

[itsjohncs]

Oh, that sounds like a solid workflow. I'll set that up, thank you @spicyj!