Update to Mocha v6 (graphql#1742)

Author: IvanGoncharov

package.json

@@ -59,11 +59,11 @@
  "beautify-benchmark": "0.2.4",
  "benchmark": "2.1.4",
  "chai": "4.2.0",
- "eslint": "5.14.0",
+ "eslint": "5.14.1",
  "eslint-plugin-flowtype": "3.4.2",
  "eslint-plugin-prettier": "3.0.1",
  "flow-bin": "0.93.0",
- "mocha": "5.2.0",
+ "mocha": "6.0.0",
  "nyc": "13.3.0",
  "prettier": "1.16.4",
  "sane": "4.0.2"