chore(package): update ava to version 0.18.1 (#17)

Author: greenkeeper[bot]

package.json

@@ -38,7 +38,7 @@
     "wordpress"
   ],
   "devDependencies": {
-    "ava": "^0.17.0",
+    "ava": "^0.18.1",
     "is-plain-obj": "^1.1.0",
     "npmpub": "^3.1.0"
   }