Bump mongodb from 2.2.36 to 3.6.1

Bumps [mongodb](https://github.com/mongodb/node-mongodb-native) from 2.2.36 to 3.6.1.
- [Release notes](https://github.com/mongodb/node-mongodb-native/releases)
- [Changelog](https://github.com/mongodb/node-mongodb-native/blob/master/CHANGES_3.0.0.md)
- [Commits](mongodb/node-mongodb-native@v2.2.36...v3.6.1)

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -7,7 +7,7 @@
   "repository": "koajs/atomic-session",
   "dependencies": {
     "csrf": "^2.0.2",
-    "mongodb": "^2.0.25",
+    "mongodb": "^3.6.1",
     "mongodb-next": "^0.7.0",
     "ms": "^2.0.0"
   },