chore(package): update ava to version 0.19.1

Closes #212

https://greenkeeper.io/

Author: greenkeeper[bot]

package.json

@@ -58,7 +58,7 @@
     "yargs": "^6.0.0"
   },
   "devDependencies": {
-    "ava": "^0.18.1",
+    "ava": "^0.19.1",
     "babel-cli": "^6.18.0",
     "babel-plugin-add-module-exports": "^0.2.1",
     "babel-plugin-transform-async-to-module-method": "^6.16.0",