Fix Gemfile.development_dependencies path in dummy app

The dummy app Gemfile was looking for Gemfile.development_dependencies three
levels up (at repo root), but after the monorepo restructure it's in
react_on_rails/Gemfile.development_dependencies (two levels up from dummy).

Changed path from ../../../ to ../../

Fixes Integration Tests CI failure where bundle was unable to find the file.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

react_on_rails/spec/dummy/Gemfile

@@ -2,6 +2,6 @@
 
 source "https://rubygems.org"
 
-eval_gemfile File.expand_path("../../../Gemfile.development_dependencies", __dir__)
+eval_gemfile File.expand_path("../../Gemfile.development_dependencies", __dir__)
 
 gem "react_on_rails", path: "../.."