Add rackup as a development dependency

Author: Ginja

mcp.gemspec

@@ -31,5 +31,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency("activesupport")
   spec.add_development_dependency("puma", ">= 5.0.0")
   spec.add_development_dependency("rack", ">= 2.0.0")
+  spec.add_development_dependency("rackup", ">= 2.1.0")
   spec.add_development_dependency("sorbet-static-and-runtime")
 end