chore: Add base64 to gemspec

[shmokmt]

Fixes

Add base64 to gemspec, which is no longer the default gem from Ruby 3.4. In fact, when I run the make test command on Ruby 3.3 I get the warning.

warning: base64 was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.4.0. Add base64 to your Gemfile or gemspec.

References

• similar support in rails/rails: https://github.com/rails/rails/pull/48907
• bugs.ruby-lang: https://bugs.ruby-lang.org/issues/19776
• ruby/ruby: https://github.com/ruby/ruby/pull/8126

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [ ] 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

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.