feat: require Node.js 6

BREAKING CHANGE: remove support for Node.js < 6

Author: pvdlg

.travis.yml

@@ -1,9 +1,8 @@
 language: node_js
 node_js:
-  - 9
+  - 10
   - 8
   - 6
-  - 4
 
 # Trigger a push build on master and greenkeeper branches + PRs build on every branches
 # Avoid double build on PRs (See https://github.com/travis-ci/travis-ci/issues/1147)

package.json

@@ -20,7 +20,7 @@
     "xo": "^0.20.0"
   },
   "engines": {
-    "node": ">=4"
+    "node": ">=6"
   },
   "files": [
     "lib",