stop supporting node v4 and v5

.travis.yml

@@ -1,5 +1,3 @@
 language: node_js
 node_js:
     - "6"
-    - "5"
-    - "4"

README.md

@@ -160,6 +160,7 @@ Here is meddleware config sample, which can be used in a standard **kraken.js**
  * **3.0.0**
     * upgrade dependencies, most notably **prom-client** to 9.0.0
     * switch to koa v2 in koa unittest
+    * only node v6 is supported (stop supporting node v4 and v5)
  * **2.1.0**
     * deprecate **excludeRoutes**, use **req.originalUrl** instead of **req.path**
  * **2.0.0**

package.json

@@ -34,5 +34,8 @@
   "repository": {
     "type": "git",
     "url": "https://github.com/jochen-schweizer/express-prom-bundle.git"
+  },
+  "engines": {
+    "node": ">=6.0.0"
   }
 }