fix: package.json to reduce vulnerabilities (#81)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/npm:debug:20170905
- https://snyk.io/vuln/npm:growl:20160721

Latest report for eggjs/egg-bin:
https://snyk.io/test/github/eggjs/egg-bin

package.json

@@ -18,7 +18,7 @@
     "globby": "^6.1.0",
     "inspector-proxy": "^1.0.0",
     "intelli-espower-loader": "^1.0.1",
-    "mocha": "^3.5.3",
+    "mocha": "^4.0.0",
     "mz-modules": "^2.0.0",
     "nyc": "^11.2.1",
     "power-assert": "^1.4.4",