chore: drop support for node v4, v6 and v9

Author: vieiralucas

.travis.yml

@@ -3,10 +3,7 @@ language: node_js
 addons:
   sauce_connect: true
 node_js:
- - 4 # to be removed 2018-04-30
- - 6 # to be removed 2019-04-01
  - 8 # to be removed 2019-12-31
- - 9 # to be removed 2018-06-30
  - lts/* # safety net; don't remove
  - node # safety net; don't remove
 

package.json

@@ -30,7 +30,7 @@
     "test": "make test"
   },
   "engines": {
-    "node": ">=4"
+    "node": ">=8"
   },
   "dependencies": {
     "assertion-error": "^1.1.0",