Upgrade node on travis (#18994)

* Upgrade to current LTS version of Node

The version of NPM with Node 6 is over a year old and frequently hangs trying to install Firebase tools.

* Temporarily comment out this condition so the install runs on Travis

* Revert "Temporarily comment out this condition so the install runs on Travis"

This reverts commit 74db9366b4b1b094369388ef50fb26f3b7fd3104.

* Pin to nodejs v8.1

.travis.yml

@@ -29,7 +29,7 @@ addons:
       - fonts-droid
 language: node_js
 node_js:
-  - "6"
+  - "8.1"
 git:
   # We rely on git tags for determining the version.
   depth: false