Update dependency ava to v6

Author: renovate[bot]

package.json

@@ -23,7 +23,7 @@
     "resolve-from": "^5.0.0"
   },
   "devDependencies": {
-    "ava": "0.25.0",
+    "ava": "6.1.0",
     "babel-cli": "6.26.0",
     "babel-preset-env": "1.7.0",
     "fs-extra": "8.1.0",