chore(package): update ava to version 0.18.2

Closes #2

https://greenkeeper.io/

Author: greenkeeper[bot]

package.json

@@ -20,7 +20,7 @@
     "dev": "npm test -- --watch"
   },
   "devDependencies": {
-    "ava": "~0.17.0",
+    "ava": "~0.18.2",
     "standard": "^8.6.0"
   }
 }