Fix all Python files to work with Python2 and Gunicorn/Heroku

Procfile

@@ -1 +1 @@
-web: gunicorn api:app --log-file -
+web: gunicorn app:app --log-file -

app.py

@@ -2,10 +2,11 @@
 
 import connexion
 
+app = connexion.App(__name__, specification_dir='./swagger/')
+app.add_api('swagger.yaml',
+            arguments={
+                'title': 'Rubber Duck Booking Tool'
+            })
+
 if __name__ == '__main__':
-    app = connexion.App(__name__, specification_dir='./swagger/')
-    app.add_api('swagger.yaml',
-                arguments={
-                    'title': 'Rubber Duck Booking Tool'
-                })
     app.run(port=8080)

controllers/default_controller.py

@@ -1,3 +1,2 @@
-
-def ducks_get() -> str:
+def ducks_get():
     return 'do some magic!'