net/bosh-bootloader: Update to 7.4.0

Release notes:

• https://github.com/cloudfoundry/bosh-bootloader/releases/tag/v7.3.0
• https://github.com/cloudfoundry/bosh-bootloader/releases/tag/v7.3.5
• https://github.com/cloudfoundry/bosh-bootloader/releases/tag/v7.4.0

Sponsored by: Pivotal Software, Inc.