Add git-secrets --scan on Travis to detect unencrypted private key.

[blueandgold]

We install git-secrets in our local environments to prevent someone from committing private keys in the local repo. But we should also have a check on Travis (--scan option), in case this gets past the local check (i.e. if someone did not install or correctly configure git-secrets).

[dekuhn]

Will keep this in 2.25 release. The 2.25 sprint is being extended by 2 weeks

[gkowalski-google]

Re-open PR#3553. Only change should be to remove the last 4 lines so that the git-secret exit code will fail the build if a secret is found. Show a failed test build with a fake secret to show this is working.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.