Add base64 as a dependency explicitly for Ruby 3.4.0 support

ref. https://www.ruby-lang.org/en/news/2023/12/25/ruby-3-3-0-released/

Author: mataku

fastimage.gemspec

@@ -22,6 +22,7 @@ Gem::Specification.new do |s|
   s.require_paths = ["lib"]
   s.rubygems_version = %q{1.3.6}
   s.summary = %q{FastImage - Image info fast}
+  s.add_dependency 'base64', '~> 0.2.0'
   s.add_development_dependency 'fakeweb-fi', '~> 1.3'
   # Note rake 11 drops support for ruby 1.9.2
   s.add_development_dependency('rake', ">= 10.5")