fix: Only require twilio_webhook_authentication if Rack version > 2

[jasonnoble]

Fixes #605

Fixes

Only require the twilio_webhook_authentication functionality if the Rack version is > 2.

FYI, #602 attempted to fix this, but doesn't work when the Rack version is < 2.

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [ ] I have added inline documentation to the code I modified

[jasonnoble]

I'm more than willing to add tests/documentation if you feel it's required. Pretty small change IMO.