test: update Travis config to use the current Node

Remove end-of-life Node.js 7 and add the current Node.js 9.

PR-URL: #307
Reviewed-By: Mykola Bilochub <nbelochub@gmail.com>
Reviewed-By: Dmytro Nechai <nechaido@gmail.com>

.travis.yml

@@ -2,8 +2,8 @@ language: node_js
 
 node_js:
   - 6
-  - 7
   - 8
+  - 9
 
 os:
   - linux