Drop Node 4, 5, and 7

.travis.yml

@@ -1,9 +1,6 @@
 language: node_js
 node_js:
-  - "4"
-  - "5"
   - "6"
-  - "7"
   # Note: docker-candidacy.sh chooses 8 as the version to run Docker builds.
   # That's pretty arbitrary, but I had to pick something. When you remove this
   # from the build matrix, be sure to adjust that script too.

package.json

@@ -126,7 +126,7 @@
   ],
   "license": "Apache-2.0",
   "engines": {
-    "node": ">=4 <=8",
+    "node": ">=6 <=9",
     "npm": ">=2"
   },
   "greenkeeper": {