chore: update engines to reflect (maintained) versions of Node being tested in Travis

BREAKING CHANGE:

Requires Node 10.

Author: brettz9

package.json

@@ -38,7 +38,7 @@
     "prettier": "^1.14.2"
   },
   "engines": {
-    "node": ">=6"
+    "node": ">=10"
   },
   "license": "ISC",
   "lint-staged": {