add dependency on bigdecimal for ruby >= 3.4

Author: toy

image_optim_rails.gemspec

@@ -23,6 +23,9 @@ Gem::Specification.new do |s|
   s.add_dependency 'image_optim', '~> 0.24'
   s.add_dependency 'railties'
   s.add_dependency 'sprockets'
+  if RUBY_VERSION >= '3.4'
+    s.add_dependency 'bigdecimal'
+  end
 
   s.add_development_dependency 'image_optim_pack', '~> 0.2', '>= 0.2.2'
   s.add_development_dependency 'rspec', '~> 3.0'