chore(package): update ava to version 0.15.0

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -36,7 +36,7 @@
     "mkdirp": "0.5.1"
   },
   "devDependencies": {
-    "ava": "^0.14.0",
+    "ava": "^0.15.0",
     "coveralls": "^2.11.9",
     "eslint": "^2.9.0",
     "eslint-config-pedant": "^0.5.1",