chore(package): update ava to version 0.21.0 (#30)

Closes #27

Author: jescalan

package.json

@@ -35,7 +35,7 @@
     "sugarss": "^1.0.0"
   },
   "devDependencies": {
-    "ava": "^0.19.1",
+    "ava": "^0.21.0",
     "coveralls": "^2.13.1",
     "nyc": "^11.0.2",
     "rewire": "^2.5.2",