Fix tests

Author: 3v0k4

test/installer_test.rb

@@ -58,6 +58,7 @@ def with_new_rails_app
           gemfile = File.read("Gemfile")
           gemfile.gsub!(/^gem "importmap-rails".*/, "")
           gemfile << %(gem "importmap-rails", path: #{File.expand_path("..", __dir__).inspect}\n)
+          gemfile << %(gem "concurrent-ruby", "1.3.4"\n) # Needed for rails < 7.1
           if Rails::VERSION::PRE == "alpha"
             gemfile.gsub!(/^gem "rails".*/, "")
             gemfile << %(gem "rails", path: #{Gem.loaded_specs["rails"].full_gem_path.inspect}\n)