chore(package): update ava to version 0.18.1

Closes #369

https://greenkeeper.io/

Author: greenkeeper[bot]

package.json

@@ -17,7 +17,7 @@
     "webpack": "1 || 2 || ^2.1.0-beta || ^2.2.0-rc"
   },
   "devDependencies": {
-    "ava": "^0.17.0",
+    "ava": "^0.18.1",
     "babel-cli": "^6.18.0",
     "babel-core": "^6.0.0",
     "babel-eslint": "^7.1.0",