require bundle in the app generator

The app generator is not generally run under bundler, but the Bundler
constant is used here.

In particular you cannot create --dev apps without this.

railties/lib/rails/generators/app_base.rb

@@ -251,6 +251,7 @@ def bundle_command(command)
         # Thanks to James Tucker for the Gem tricks involved in this call.
         _bundle_command = Gem.bin_path('bundler', 'bundle')
 
+        require 'bundler'
         Bundler.with_clean_env do
           print `"#{Gem.ruby}" "#{_bundle_command}" #{command}`
         end