run node 8 in both travis and now (#346)

* run node 10 in both travis and now

* on second thought, use node v8 everywhere

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 node_js:
-  - "9"
+  - "8"
 dist: trusty   # needed for chrome headless
 sudo: required # needed for chrome headless
 addons:

package.json

@@ -164,7 +164,10 @@
       "inline-script.js",
       "webpack.client.config.js",
       "webpack.server.config.js"
-    ]
+    ],
+    "engines": {
+      "node": "^8.0.0"
+    }
   },
   "greenkeeper": {
     "ignore": [