chore(package): update ava to version 0.15.0

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -36,7 +36,7 @@
     "sourceMaps": "inline"
   },
   "devDependencies": {
-    "ava": "^0.14.0",
+    "ava": "^0.15.0",
     "babel-cli": "^6.6.5",
     "babel-preset-es2015": "^6.6.0",
     "babel-preset-stage-1": "^6.5.0",