api-docs is currently affected by the high-severity [ReDOS vulnerability](https://snyk.io/vuln/npm:negotiator:20160616). 

Vulnerable module: `negotiator`
Introduced through: ` express`

This PR fixes the ReDoS vulnerability by upgrading `express` to version 4.0.0.

Check out the [Snyk test report](https://snyk.io/test/github/maxcdn/api-docs) to review other vulnerabilities that affect this repo. 

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Stay secure, 
The Snyk team