Pin FFI gem version to 1.12.2 for Ruby 2.0 compatibility in Traffic Portal builder image

[zrhoffman]

What does this PR (Pull Request) do?

The FFI gem updated to version 1.13.0 yesterday, which included raising the minimum Ruby version to 2.3, which is higher than the version that CentOS 7 provides by default (2.0).

• This PR is not related to any Issue

Which Traffic Control components are affected by this PR?

• Traffic Portal
• Build Environment

What is the best way to verify this PR?

In the build docker-compose.yml:

• Comment out the Traffic Portal image: line
• Uncomment the Traffic Portal build: lines

Run pkg -v traffic_portal_build and verify that the expected Traffic Portal RPMs are produced

If this is a bug fix, what versions of Traffic Control are affected?

• master (deadd96)
• 4.0.0
• 4.1.0 (RC1)

The following criteria are ALL met by this PR

• Build environment has no tests
• This PR includes documentation in the form of comments
• An update to CHANGELOG.md is not necessary
• This PR includes any and all required license headers
• This PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information

[mitchell852]

looks good. TP RPM built w/ no problems. I will merge once the auto checks are complete.