Fix "Deploy to Heroku" button

Prior to this patch, Gavel crashed when run on Heroku because there was
no 'config.yaml' file present.

README.md

@@ -63,6 +63,9 @@ the required settings (the ones that don't have default values).
 Most settings can either be set in `config.yaml` or set as environment
 variables. There's more detailed documentation in `config.sample.yaml`.
 
+If you don't want to use the config file and use only environment variables,
+set the environment variable `IGNORE_CONFIG_FILE=true`.
+
 ## Use
 
 Using the admin interface on `/admin`, input data for all the projects and

app.json

@@ -5,7 +5,8 @@
     "description": "An awesome judging system for hackathons",
     "env": {
         "ADMIN_PASSWORD": "change-this-before-deploying",
-        "SECRET_KEY": "randomly-generate-this-before-deploying"
+        "SECRET_KEY": "randomly-generate-this-before-deploying",
+        "IGNORE_CONFIG_FILE": "true"
     },
     "keywords": [
         "gavel",

gavel.py

@@ -30,6 +30,7 @@
 app = Flask(__name__)
 app.config['SQLALCHEMY_DATABASE_URI'] = DB_URI
 app.config['SECRET_KEY'] = SECRET_KEY
+db.app = app
 db.init_app(app)
 
 @app.route('/')

settings.py

@@ -7,8 +7,11 @@
 class Config(object):
 
     def __init__(self, config_file):
-        with open(config_file) as f:
-            self._config = yaml.safe_load(f)
+        if not os.environ.get('IGNORE_CONFIG_FILE', False):
+            with open(config_file) as f:
+                self._config = yaml.safe_load(f)
+        else:
+            self._config = {}
 
     # checks for an environment variable first, then an entry in the config file,
     # and then falls back to default