fix: package.json to reduce vulnerabilities (#2)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/npm:ms:20170412

Latest report for koajs/atomic-session:
https://snyk.io/test/github/koajs/atomic-session

package.json

@@ -9,7 +9,7 @@
     "csrf": "^2.0.2",
     "mongodb": "^2.0.25",
     "mongodb-next": "^0.7.0",
-    "ms": "^0.7.0"
+    "ms": "^2.0.0"
   },
   "devDependencies": {
     "istanbul": "0",