chore(package): update ava to version 3.8.1

Closes #26

Author: greenkeeper[bot]

package.json

@@ -25,7 +25,7 @@
     "rxjs": ">=6.0.0"
   },
   "devDependencies": {
-    "ava": "2.2.0",
+    "ava": "3.8.1",
     "eslint": "6.1.0",
     "eslint-config-es2015": "1.1.0",
     "eslint-config-prettier": "6.0.0",