ci(travis): upgrade to node8

Author: janpio

.travis.yml

@@ -12,12 +12,12 @@ addons:
 env:
   global:
     - SAUCE_USERNAME=snay
-    - TRAVIS_NODE_VERSION=6
+    - TRAVIS_NODE_VERSION=8
     - ANDROID_API_LEVEL=28
     - ANDROID_BUILD_TOOLS_VERSION=28.0.3
 
 language: node_js
-node_js: 6
+node_js: 8
 
 # yaml anchor/alias: https://medium.com/@tommyvn/travis-yml-dry-with-anchors-8b6a3ac1b027