chore(deps): update dependency mocha to v6 (#430)

Author: renovate[bot]

packages/packages/google-gax/package.json

@@ -47,7 +47,7 @@
     "eslint-plugin-prettier": "^3.0.0",
     "gts": "^0.9.0",
     "intelli-espower-loader": "^1.0.1",
-    "mocha": "~5.2.0",
+    "mocha": "~6.0.0",
     "nyc": "^13.1.0",
     "pegjs": "~0.10.0",
     "prettier": "^1.15.2",

packages/packages/google-gax/samples/package.json

@@ -14,6 +14,6 @@
     "google-gax": "^0.25.2"
   },
   "devDependencies": {
-    "mocha": "^5.2.0"
+    "mocha": "^6.0.0"
   }
 }