Remove a bunch of scary warnings around Git

* Set name and email in gitconfig so students can actually `git commit`; otherwise they can't commit at all
* Automatically trust Heroku's SSH key, to prevent a warning about "The authenticity of the host can't be established"
* Set `push.default = current` in gitconfig so `git push heroku` (vs `git push heroku master`) works without an error
@decklin's previous comment:

> Well, `vagrant up` should re-run the provison, which *should* be idempotent, but will definitely re-run more than it strictly needs to. I'll have to think about this.