ENGCOM-1453: chore: upgrade Node.js to 8 #15018

Author: VladimirZaets

.travis.yml

@@ -19,7 +19,7 @@ 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