chore: upgrade Node.js to 8

Author: DanielRuf

.travis.yml

@@ -22,7 +22,7 @@ env:
   global:
     - COMPOSER_BIN_DIR=~/bin
     - INTEGRATION_SETS=3
-    - NODE_JS_VERSION=6
+    - NODE_JS_VERSION=8
     - MAGENTO_HOST_NAME="magento2.travis"
   matrix:
     - TEST_SUITE=unit