Use built-in Heroku support over our custom support.

[cgc]

The complicated startup solution that I wrote last year should actually be superseded by psiTurk's builtin support for Heroku. I think the only change we'd need to add to the README is heroku config:set ON_CLOUD=1 (though b/c we use an older psiTurk version, it may require ON_HEROKU), though I'd want to read the docs and read the script more closely to ensure that's the case. We wouldn't want to directly run those things since they create new files (like Procfile and requirements).

[fredcallaway]

This is a case where "if it ain't broke, don't fix it" overrules "there should be one and only one right way to do it" (for me). But if you want to implement and test this, go ahead!