Leave Rackup as a dev dependancy

It's only used for a Lobster server whilst testing.

Author: shamess

ruby/umbra-rb.gemspec

@@ -33,13 +33,13 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "pry"
   spec.add_development_dependency "rake", "~> 13.0"
   spec.add_development_dependency "rspec", "~> 3.0"
+  spec.add_development_dependency "rackup", ">=2", "< 3"
   spec.add_development_dependency "rack-test"
   spec.add_development_dependency "puma"
 
   spec.add_dependency "concurrent-ruby", "~> 1.1"
   spec.add_dependency "redis", "~> 4.1"
   spec.add_dependency "google-protobuf", "~> 3"
   spec.add_dependency "rack", ">= 2", "< 4"
-  spec.add_dependency "rackup", ">=2", "< 3"
   spec.add_dependency "zeitwerk", "~> 2"
 end