chore(package): update ava to version 0.15.0

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -60,7 +60,7 @@
     "postcss": "^5.0.21"
   },
   "devDependencies": {
-    "ava": "^0.14.0",
+    "ava": "^0.15.0",
     "ava-codemods": "^0.2.1",
     "babel-cli": "^6.9.0",
     "babel-eslint": "^6.0.4",