Add secret key generate to travis ci build to support forks

[rich-allen-gov]

Checklist

I have…

• run the application locally (bin/rails server) and verified that my changes behave as expected.
• run static code analysis (bin/rubocop) and vulnerability scan (bin/brakeman) against my changes.
• run the test suite (bin/rake spec) and verified that all tests pass.
• summarized below my changes and noted which issues (if any) this pull request fixes or addresses.
• thoroughly outlined below the steps necessary to test my changes.
• attached screenshots illustrating relevant behavior before and after my changes.
• read, understand, and agree to the terms described in CONTRIBUTING.md.
• added my name, email address, and copyright date to CONTRIBUTORS.md.

Summary of Changes

If you want to work with a fork of the project, the build fails for pull requests, most likely due to this limitation. https://docs.travis-ci.com/user/pull-requests/#Pull-Requests-and-Security-Restrictions and not having the SECRET_KEY_BASE environment variable set up. This is a small change to the travis build which just generates a secret key base at build time.

Testing

1. In travis ci delete the SECRET_KEY_BASE variable.
2. Run the build. You should not receive errors related to not having that variable set.

Link with behavior:

before:
https://travis-ci.org/deptofdefense/move.mil/builds/352455122

after:
This pull request should pass testing with no secret key base env var needed in configuration.