Update travis node versions with minimal version 6

Author: wimhendrikx

.travis.yml

@@ -1,6 +1,11 @@
 sudo: false
 language: node_js
 node_js:
-  - stable
-  - "6"
-  - "4"
+  - 6
+  - 7
+  - 8
+  - 9
+  - 10
+  - 11
+  - 12
+  - node