fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/npm:debug:20170905
- https://snyk.io/vuln/npm:growl:20160721
- https://snyk.io/vuln/npm:mime:20170907

Latest report for donmclean/riko:
https://snyk.io/test/github/donmclean/riko

package.json

@@ -94,7 +94,7 @@
     "less": "^2.7.1",
     "less-loader": "^2.2.3",
     "lodash": "^4.15.0",
-    "mocha": "^3.0.2",
+    "mocha": "^4.0.0",
     "mocha-lcov-reporter": "^1.2.0",
     "morgan": "^1.7.0",
     "node-sass": "^4.5.3",
@@ -112,7 +112,7 @@
     "stylus": "^0.54.5",
     "stylus-loader": "^3.0.1",
     "update-notifier": "^2.2.0",
-    "url-loader": "^0.5.7",
+    "url-loader": "^0.6.0",
     "webpack": "2.3.1",
     "webpack-dev-server": "^2.6.1",
     "webpack-merge": "^4.1.0",