build: update travis to use most recent nodejs LTS version (#13104)

Author: devversion

.travis.yml

@@ -5,7 +5,7 @@ sudo: false
 node_js:
   # Use the explicit NodeJS LTS version 8.9.4 to avoid any automatic upgrade of the version.
   # This ensures that all Travis jobs run consistently and don't have different Node versions.
-  - '8.11.3'
+  - '8.12.0'
 
 addons:
   jwt: