fix: package.json to reduce vulnerabilities (#4)

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

Latest report for koajs/resourcer:
https://snyk.io/test/github/koajs/resourcer

package.json

@@ -16,7 +16,7 @@
  "author": "Aaron Heckmann <[email protected]>",
  "license": "MIT",
  "dependencies": {
- "debug": "1.0.4",
+ "debug": "2.6.9",
  "koa-mount": "1.3.0"
  },
  "devDependencies": {