Make payment providers optional

[victorferreira]

- Do you want to request a feature or report a bug? Bug

- What is the current behavior? App crashes on first launch.

2019-01-29T11:30:52.336109+00:00 app[web.1]: time="2019-01-29T11:30:52Z" level=fatal msg="Error loading instance config: No payment providers enabled

- If the current behavior is a bug, please provide the steps to reproduce. Deploy a new application to Heroku using the "Deploy to Heroku" button.

- What is the expected behavior? App should lunch after deployed.

- Please mention your Go version, and operating system version. Just using the standard Heroku buildpack.

I know it says the a I don't have payment provider enabled on the log, but here's a few things to consider:

• The README doesn't list the Payment section as required
• Even after adding PAYMENT_STRIPE_ENABLED=true and restarting the server, the app still does't launch.

[bcomnes]

Agreed, app should launch even if payment providers are not configured.