Update travis node js env version from 4 to 6

Philipp Alferov · Philipp Alferov · commit d8c578376a9e · 2018-08-20T15:35:06.000-04:00
diff --git a/.travis.yml b/.travis.yml
@@ -7,7 +7,7 @@ notifications:
   email: false
 node_js:
   - 'stable'
-  - '4'
+  - '6'
 branches:
   except:
     - "/^v\\d+\\.\\d+\\.\\d+$/"
