Update dev-dependencies

Author: wooorm

package.json

@@ -42,7 +42,7 @@
     "remark-preset-wooorm": "^2.0.0",
     "tape": "^4.0.0",
     "unist-builder": "^1.0.1",
-    "xo": "^0.17.0"
+    "xo": "^0.18.0"
   },
   "scripts": {
     "build-md": "remark . --quiet --frail --output",
@@ -56,6 +56,7 @@
   },
   "xo": {
     "space": true,
+    "esnext": false,
     "rules": {
       "guard-for-in": "off",
       "eqeqeq": "off",