feat(node): update to node 8 (#276)

.travis.yml

@@ -26,7 +26,7 @@ notifications:
     on_failure: change
 
 node_js:
-  - "6"
+  - "8"
 
 addons:
   apt:

package.json

@@ -20,7 +20,7 @@
   "readmeFilename": "README.md",
   "homepage": "https://github.com/mozilla/fxa-js-client",
   "engines": {
-    "node": ">=6"
+    "node": ">=8"
   },
   "repository": {
     "type": "git",