chore(package): update ava to version 0.20.0

Author: greenkeeper[bot]

package.json

@@ -37,7 +37,7 @@
     "lodash.isstring": "^4.0.1"
   },
   "devDependencies": {
-    "ava": "^0.18.1",
+    "ava": "^0.20.0",
     "babel-eslint": "^7.1.1",
     "eslint": "^3.10.2",
     "eslint-config-iansinnott": "^1.0.1",