chore(package): update mocha to version 6.0.0

Author: greenkeeper[bot]

package.json

@@ -26,6 +26,6 @@
   "devDependencies": {
     "chai": "^4.0.2",
     "fbp-spec": "^0.6.6",
-    "mocha": "^5.0.0"
+    "mocha": "^6.0.0"
   }
 }