specify engines version in package.json for heroku

package.json

@@ -135,8 +135,9 @@
     "sinon-chai": "^2.13.0"
   },
   "engines": {
-    "node": "6.x",
-    "npm": "5.x"
+    "node": "6.11.3",
+    "npm": "5.4.2",
+    "yarn": "1.1.0"
   },
   "config": {
     "blanket": {