configure travis to support nodejs v6.12.3+

Author: ech0s7r

.travis.yml

@@ -4,6 +4,9 @@ git:
 matrix:
   fast_finish: true
   include:
+    - node_js: 6.12.3
+    - node_js: 6
+    - node_js: 7
     - node_js: 8
     - node_js: 9
     - node_js: 10