Remove node 4 support

.travis.yml

@@ -1,7 +1,6 @@
 language: node_js
 node_js:
 - '6'
-- '4'
 
 sudo: false
 
@@ -18,4 +17,4 @@ branches:
   - master
   - /^v\d+\.\d+\.\d+/
 
-script: npm run test
+script: npm run test

package.json

@@ -24,6 +24,9 @@
     "url": "https://github.com/electron/electron-rebuild/issues"
   },
   "homepage": "https://github.com/electron/electron-rebuild",
+  "engines": {
+    "node": ">=6.0.0"
+  },
   "dependencies": {
     "babel-register": "^6.16.3",
     "babel-runtime": "6.18.0",