Revert Gemfile path change - simpler is better

The File.join(destination_root, "Gemfile") approach added unnecessary
complexity. Rails generators run with the Rails root as the working
directory, so the simple "Gemfile" string is more reliable and
consistent with how other Rails generators resolve paths.

This leaves NO net changes from this PR since the auto-remove feature
was already reverted in commit 24c4c9c.

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

lib/generators/react_on_rails/install_generator.rb

@@ -203,7 +203,7 @@ def shakapacker_in_bundler_specs?(gem_name)
       end
 
       def shakapacker_in_gemfile_text?(gem_name)
-        gemfile = ENV["BUNDLE_GEMFILE"] || File.join(destination_root, "Gemfile")
+        gemfile = ENV["BUNDLE_GEMFILE"] || "Gemfile"
 
         File.file?(gemfile) &&
           File.foreach(gemfile).any? { |l| l.match?(/^\s*gem\s+['"]#{Regexp.escape(gem_name)}['"]/) }