feat: drop support of Node.js 4

BREAKING CHANGE:

Node.js>=6 is required

.travis.yml

@@ -1,9 +1,8 @@
 language: node_js
 node_js:
-  - 4
   - 6
   - 8
-  - 9
+  - 10
 sudo: false
 before_install:
   - curl -L https://unpkg.com/@pnpm/self-installer | node

package.json

@@ -8,7 +8,7 @@
     "lib"
   ],
   "engines": {
-    "node": ">=4"
+    "node": ">=6"
   },
   "scripts": {
     "lint": "tslint -c tslint.json --project .",