Disable node 6 (microsoft#29832)

* Disable node 6

It exits LTS in a couple of months, and doesn't support async/await,
meaning that it blocks us from switching Travis to use gulp instead of
jake.

* Swap in node 8 for node 6

Author: sandersn

.travis.yml

@@ -3,7 +3,7 @@ language: node_js
 node_js:
   - 'node'
   - '10'
-  - '6'
+  - '8'
 
 sudo: false