运行rake doc:app 报错：RubyToken::AlreadyDefinedToken (NameError)

运行rake doc:app 报错：

rm -r doc/app
rake aborted!
uninitialized constant RubyToken::AlreadyDefinedToken

(See full trace by running task with --trace)

解决方式：

在Gemfile文件中去掉 gem 'ruby-debug'