chore(ci): use Node.js by default

Author: blond

.travis.yml

@@ -5,8 +5,8 @@ language: node_js
 matrix:
   include:
     - node_js: "4"
-      env: COVERALLS=1
     - node_js: "6"
+      env: COVERALLS=1
 
 after_success:
   - if [ "x$COVERALLS" = "x1" ]; then npm run coveralls; fi