Bug 1343932 - Update nodejs used on Heroku/Travis from 6.2.0 to 7.7.1

The npm shrinkwrap was also updated since new nodejs versions come with
new npm, and the format for the shrinkwrap sometimes changes:
https://treeherder.readthedocs.io/common_tasks.html#updating-packages-in-package-json

.travis.yml

@@ -36,7 +36,7 @@ matrix:
     - env: ui-tests
       sudo: false
       language: node_js
-      node_js: "6.2.0"
+      node_js: "7.7.1"
       cache:
         # Note: This won't re-use the same cache as the linters job,
         # since caches are tied to the language/version combination.

package.json

@@ -7,7 +7,7 @@
   },
   "license": "MPL-2.0",
   "engines": {
-    "node": "6.2.0"
+    "node": "7.7.1"
   },
   "dependencies": {
     "babel-preset-es2015": "6.18.0",