Bump log4js from 6.1.0 to 6.4.0

Bumps [log4js](https://github.com/log4js-node/log4js-node) from 6.1.0 to 6.4.0.
- [Release notes](https://github.com/log4js-node/log4js-node/releases)
- [Changelog](https://github.com/log4js-node/log4js-node/blob/master/CHANGELOG.md)
- [Commits](log4js-node/log4js-node@v6.1.0...v6.4.0)

---
updated-dependencies:
- dependency-name: log4js
  dependency-type: direct:production
...

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

Author: dependabot[bot]

package-lock.json

@@ -30,7 +30,7 @@
     "file": "0.2.x",
     "glob": "^7.1.6",
     "js-yaml": "^3.13.1",
-    "log4js": "^6.1.0",
+    "log4js": "^6.4.0",
     "mustache": "^3.0.1",
     "nconf": "^0.11.4",
     "optimist": "^0.6.0",