Upgrade ajv 6.7.0 -> 6.8.1

Author: ehmicky

package-lock.json

@@ -28,7 +28,7 @@
     "lib": "src"
   },
   "dependencies": {
-    "ajv": "^6.7.0",
+    "ajv": "^6.8.1",
     "chalk": "^2.4.2",
     "cli-truncate": "^1.1.0",
     "cross-fetch": "^3.0.0",