chore(package): update ava to version 0.20.0

Author: greenkeeper[bot]

package.json

@@ -50,7 +50,7 @@
     "url": "^0.11.0"
   },
   "devDependencies": {
-    "ava": "^0.19.0",
+    "ava": "^0.20.0",
     "babel-cli": "^6.16.0",
     "babel-core": "^6.22.0",
     "babel-eslint": "^7.0.0",