fix: package.json to reduce vulnerabilities

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

Latest report for slysterous/workdates:
https://snyk.io/test/github/slysterous/workdates

package.json

@@ -30,7 +30,7 @@
   },
   "homepage": "https://github.com/slysterous/workdates#readme",
   "dependencies": {
-    "moment": "2.18.1"
+    "moment": "2.19.3"
   },
   "devDependencies": {
     "chai": "4.0.2",