Add Heroku-related stuff

[amureki]

Things needed for full Heroku integration like:

• runtime.txt
• Procfile
• app.json

[codingjoe]

You don't need a runtime.txt if you use Pipenv.

[amureki]

@codingjoe
Then it comes to the question if we are going to use pipenv by default. :)

It should be fine in terms of speed unless you have a big monolith. 😉
The only concern I see, pyup is not fully supporting pipenv (freezing packages), which can be a problem if we are going with pyup.

[codingjoe]

Then it comes to the question if we are going to use pipenv by default. :)

I know ;)