Add base64 as a runtime dependency to fix the LoadError in Ruby 3.4

[taketo1113]

This pull request has been created to address the LoadError for the base64 gem in Ruby 3.4.

This error occurs because base64 is no longer included in the default gems starting from Ruby 3.4.
https://bugs.ruby-lang.org/issues/19351

The error message is following:

irb(main):001> require 'em-websocket'
/my-app/tmp/ruby/3.4.0+0/gems/em-websocket-0.5.3/lib/em-websocket.rb:14: warning: base64 was loaded from the standard library, but is not part of the default gems starting from Ruby 3.4.0.
You can add base64 to your Gemfile or gemspec to silence this warning.
/Users/taketo/.rbenv/versions/3.4-dev/lib/ruby/3.4.0+0/bundled_gems.rb:76:in 'Kernel.require': cannot load such file -- base64 (LoadError)
	from /Users/taketo/.rbenv/versions/3.4-dev/lib/ruby/3.4.0+0/bundled_gems.rb:76:in 'block (2 levels) in Kernel#replace_require'
	...

• ruby: 3.4-dev (ruby 3.4.0dev (2024-10-29T07:56:32Z master 21b3dfa03b) +PRISM [arm64-darwin23])
• em-websocket: 0.5.3

Details

This Pull Request changes to add base64 as a runtime dependency to fix the LoadError in Ruby 3.4. (Fix #160)

Additionally, the required_ruby_version has been added to the gemspec to restrict the Ruby version to 2.4.0 or higher.
This is to explicitly specify compatibility, as the base64 gem is only available for Ruby 2.4 and above.
https://github.com/ruby/base64/blob/v0.2.0/base64.gemspec#L17