chore: drop support for Node 4

BREAKING CHANGE: This commit requires Node >=6.

Author: macklinu

.travis.yml

@@ -3,7 +3,6 @@ node_js:
 - 10
 - 8
 - 6
-- 4
 before_install:
 - curl -o- -L https://yarnpkg.com/install.sh
 - export PATH="$HOME/.yarn/bin:$PATH"

package.json

@@ -21,7 +21,7 @@
     "index.js"
   ],
   "engines": {
-    "node": ">= 4"
+    "node": ">=6"
   },
   "peerDependencies": {
     "eslint": ">=5"