chore(package): update ava to version 0.19.0

Closes #2

https://greenkeeper.io/

Author: greenkeeper[bot]

package.json

@@ -32,6 +32,6 @@
     "visa"
   ],
   "devDependencies": {
-    "ava": "^0.16.0"
+    "ava": "^0.19.0"
   }
 }