only support node versions which node still supports

Author: stefanpenner

.travis.yml

@@ -1,6 +1,5 @@
 language: node_js
 node_js:
-  - "4"
   - "6"
   - "8"
   - "10"

appveyor.yml

@@ -1,6 +1,5 @@
 environment:
   matrix:
-  - nodejs_version: "4"
   - nodejs_version: "6"
   - nodejs_version: "8"
   - nodejs_version: "10"

package.json

@@ -43,6 +43,6 @@
     "request": "^2.75.0"
   },
   "engines": {
-    "node": ">= 4"
+    "node": "6.* || >= 8.*"
   }
 }