Add base64 gem explicitly as dependency

[mataku]

Summary

closes #256

base64 is no longer the default gem from Ruby 3.4
https://www.ruby-lang.org/en/news/2023/12/25/ruby-3-3-0-released/

On Ruby 3.3, we could see 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.

From Ruby 3.4, we will get the error:

Failure/Error: require 'base64'

LoadError:
  cannot load such file -- base64

---

Added

Add base64 dependency to gemspec

[mataku]

@swarley please review and let me know if you have any questions

[PixeLInc]

This fix was included in another PR that was merged: #277

so going to close this one out. Thanks for the PR, though.