fix: latest version of `dd-trace` broke the gem

[duncanista]

What does this PR do?

Upgrades dd-trace to 1.15.0.

Motivation

Users installing the gem can bring their own tracer. dd-trace-rb#3150 made breaking changes for us.

Testing Guidelines

Updated snapshots.

Additional Notes

Types of changes

• [x] Bug fix
• [ ] New feature
• [ ] Breaking change
• [x] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [ ] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog

[muzfuz]

Thank you for fixing this, I was just about to raise an issue 😄