Add psycopg2 to Pipfile for SQLAlchemy Postgresql support

I'm not sure why this isn't installed automatically, but I guess in order to make sure tons of unused dependencies aren't installed on machines that don't need them as well as all the errors that crop up, it wasn't added.

As a user who's simply using it for deployment and didn't need it for local development, I suppose this is what happens.

app.py

@@ -8,7 +8,6 @@ from faker import Faker
 from flask import Flask, render_template, request
 from flask_login import LoginManager, current_user
 from flask_sqlalchemy import SQLAlchemy
-# init SQLAlchemy
 from werkzeug.security import generate_password_hash
 
 db = SQLAlchemy()
@@ -23,6 +22,7 @@ def create_app():
         app.config['SECRET_KEY'] = 'secret key goes here'
         app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///db.sqlite'
 
+    # Heroku deployment
     if app.config['ENV'] == 'production':
         app.config['SECRET_KEY'] = os.getenv('SECRET_KEY')
         app.config['SQLALCHEMY_DATABASE_URI'] = os.getenv('DATABASE_URL').replace('postgres://', 'postgresql://', 1)