fix: package.json to reduce vulnerabilities (#179)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/npm:moment:20161019

Latest report for pa11y/dashboard:
https://snyk.io/test/github/pa11y/dashboard

package.json

@@ -30,7 +30,7 @@
     "express": "~4.14",
     "express-hbs": "~1.0",
     "http-headers": "^3.0.1",
-    "moment": "~2.13",
+    "moment": "~2.15.2",
     "pa11y-webservice": "^2.3.0",
     "pa11y-webservice-client-node": "^1.2.1",
     "underscore": "~1.8"