fixed heroku build process so it skips building the frontend

Procfile

@@ -1 +1 @@
-web: npm install --prefix backend && npm start --prefix backend
+web: npm start --prefix backend

package.json

@@ -6,6 +6,7 @@
     "ng": "ng",
     "start": "ng serve",
     "build": "ng build",
+    "heroku-postbuild": "npm install --prefix backend",
     "test": "ng test",
     "lint": "ng lint",
     "e2e": "ng e2e",