Make node.js 6 the minimum supported version.

.travis.yml

@@ -4,7 +4,7 @@ language: node_js
 git:
     depth: 10
 node_js:
-    - "4"
+    - "6"
     - "8"
 before_install:
     - if [[ `npm -v` != 5* ]]; then npm install -g npm@5; fi

package.json

@@ -78,7 +78,7 @@
     "./src/location.js": false
   },
   "engines": {
-    "node": ">=4"
+    "node": ">=6"
   },
   "files": [
     "bin",