Add base64 as a dependency explicitly for Ruby 3.4.0 support

[mataku]

Context

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

Changes

add base64 gem as a dependency to gemspec