Remove `mime-types`, `rexml`, and `webrick` runtime dependencies

[orien]

Summary

I notice cucumber-rails doesn't directly use the mime-types, rexml, and webrick gems.

Let's remove them from the gemspec to avoid forcing unnecessary dependencies on downstream projects.

How Has This Been Tested?

The automated test suite passes showing these gems are unneeded.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue).
• [ ] New feature (non-breaking change which adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• [ ] I've added tests for my code.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.

[aslakhellesoy]

Hi @orien,

Thanks for your making your first contribution to Cucumber, and welcome to the Cucumber committers team! You can now push directly to this repo and all other repos under the cucumber organization! 🍾

In return for this generous offer we hope you will:

• ✅ Continue to use branches and pull requests. When someone on the core team approves a pull request (yours or someone else's), you're welcome to merge it yourself.
• 💚 Commit to setting a good example by following and upholding our code of conduct in your interactions with other collaborators and users.
• 💬 Join the community Slack channel to meet the rest of the team and make yourself at home.
• ℹ️ Don't feel obliged to help, just do what you can if you have the time and the energy.
• 🙋 Ask if you need anything. We're looking for feedback about how to make the project more welcoming, so please tell us!

On behalf of the Cucumber core team, Aslak Hellesøy Creator of Cucumber