Integrate brakeman security scans to our Travis CI checks

[edwinthinks]

Summary

We should run security scans to our TravisCI checks to better ensure we don't introduce security vulnerabilities. Let's update our Travis configuration to also run these checks and let us know if we should fix any vulnerabilities.

https://brakemanscanner.org/docs/quickstart/

Things to Consider

We should add this also to the partner application https://github.com/rubyforgood/partner

Criteria for Completion

• TravisCI tests should fail if a pull request has a security vulnerability