Specify RBS requirement during export command

Author: KaanOzkan

lib/spoom/cli/srb/sigs.rb

@@ -145,6 +145,7 @@ class Module; include T::Sig; end
           tapioca_context.write!("Gemfile", <<~RB)
             source "https://rubygems.org"
 
+            gem "rbs", ">= 4.0.0.dev.4"
             gem "tapioca"
             gem "#{spec.name}", path: "#{copy_context.absolute_path}"
           RB