drop node 4 support

.travis.yml

@@ -3,6 +3,5 @@ language: node_js
 node_js:
   - "8"
   - "6"
-  - "4"
 after_success:
   - npm run coveralls

appveyor.yml

@@ -8,7 +8,6 @@ environment:
   matrix:
     - nodejs_version: "8"
     - nodejs_version: "6"
-    - nodejs_version: "4"
 
 install:
   - ps: Install-Product node $env:nodejs_version

package.json

@@ -68,6 +68,6 @@
     ]
   },
   "engines": {
-    "node": ">= 4"
+    "node": ">= 6"
   }
 }