Use In Memory SQLite DB rather than Postgres

[eriktaubeneck]

Since I usually use this example for testing and working on Flask-Social as a minimal implementation, I usually find it easier to just work with an in memory SQLite DB. By simply removing the SQLALCHEMY_DATABASE_URI from the config/app.yml file, this happens automatically. I would then suggest adding the line to the PRODUCTION config section, as an example of how to use it in production.

@mattupstate Easy enough to make the change and provide a pull request if you think it's a worthwhile change.