Fix tests in railties

Since `bundle install` was fixed in `rails plugin new`, it
now requires `rails 4.0.0.beta` version in filesystem when
running tests. Instead of providing it, we can run tested
command with `--dev` option, to use rails from the local
directory.

railties/test/generators/plugin_new_generator_test.rb

@@ -99,7 +99,7 @@ def test_ensure_that_database_option_is_passed_to_app_generator
   end
 
   def test_generation_runs_bundle_install_with_full_and_mountable
-    result = run_generator [destination_root, "--mountable", "--full"]
+    result = run_generator [destination_root, "--mountable", "--full", "--dev"]
     assert_file "#{destination_root}/Gemfile.lock" do |contents|
       assert_match(/bukkits/, contents)
     end