Bump node version

.travis.yml

@@ -1,8 +1,6 @@
 language: node_js
-node_js: 8
-cache:
-  directories:
-    - node_modules
+node_js: 10
+cache: npm
 before_script: node -r esm index.js &
 after_success: npx apollo schema:check
 deploy:

package.json

@@ -1,6 +1,6 @@
 {
   "engines": {
-    "node": "8"
+    "node": "10"
   },
   "scripts": {
     "pretest": "eslint index.js",