Update Getting Started App

[CaseyFaist]

This PR updates:

Python -> 3.7.0
Django -> 2.1

This also reverts the requirement management to a version-specific requirements.txt file.

Instructions have been updated to reflect changes.

[spookyuser]

Any reasons for the changing to requirements.txt?

[pkrefta]

What's the reason behind removing pipenv support ? It's very simple to use specific package versions in Pipfile

[CaseyFaist]

Awesome question! I'm actually interested in providing more explicit guides about environment managers, Pipenv especially, and how the buildpack uses those systems differently than req.txt - the env management landscape is pretty interesting in Python right now. See: http://andrewsforge.com/article/python-new-package-landscape/.

This is a getting started guide, so I figured it's good to start with the basics - you don't actually need an environment to deploy to Heroku, it's just a good idea to have - and create guides for how to migrate projects away from req.txt and use local env managers with Heroku explicitly, so developers with less experience with package management have more context on what problems Pipenv is solving and how it's different than some other options out there, like Conda, venv and others.

So expect some more detailed Pipenv info coming up! If there are other environment managers you want to see documented on how to use with Heroku as well, be sure to let me know 👍