chore(package): update ava to version 3.7.1

Closes #68

Author: greenkeeper[bot]

package.json

@@ -31,7 +31,7 @@
     "update-notifier": "^4.0.0"
   },
   "devDependencies": {
-    "ava": "^2.3.0",
+    "ava": "^3.7.1",
     "babel-preset-env": "^1.5.2",
     "husky": "^3.0.0",
     "semantic-release": "^15.0.0",