Add a Ruby head test configuration

ruby/google-protobuf.gemspec

@@ -32,6 +32,8 @@ Gem::Specification.new do |s|
     s.add_development_dependency "rake-compiler-dock", "= 1.2.1"
   end
   s.required_ruby_version = '>= 3.0'
+  # bigdecimal must be used as a non-built in gem as of ruby-3.4
+  s.add_dependency "bigdecimal"
   # TODO: evaluate removing Rakefile and moving logic to extconf.rb, so that we
   # can remove this runtime dependency on rake. See the discussion here for
   # more details:
@@ -41,6 +43,4 @@ Gem::Specification.new do |s|
   s.add_development_dependency "ffi-compiler", "~>1"
   s.add_development_dependency "rake-compiler", "~> 1.1.0"
   s.add_development_dependency "test-unit", '~> 3.0', '>= 3.0.9'
-  # bigdecimal must be used as a non-built in gem as of ruby-3.4
-  s.add_development_dependency "bigdecimal" if RUBY_PLATFORM >= '3.4.0'
 end