Use heroku-postbuild instead of postinstall.

package.json

 {
     "name": "Opentech",
     "version": "1.0.0",
-    "description": "Build the static src.",
-    "cacheDirectories": ["client/node_modules"],
+    "description": "Build the static_src assets.",
+    "cacheDirectories": ["opentech/static_src/node_modules"],
     "scripts": {
-        "postinstall": "cd opentech/static_src && yarn install && npm run build:prod"
+        "heroku-postbuild": "cd opentech/static_src && yarn install && npm run build:prod"
     },
     "engines": {
         "node": "8.11.3",

requirements.txt

@@ -29,7 +29,7 @@ django_select2==6.0.1
 
 # Production dependencies
 dj-database-url==0.5.0
-django-heroku=0.3.1
+django-heroku==0.3.1
 whitenoise==2.0.4
 gunicorn==19.9.0
 ConcurrentLogHandler==0.9.1