如何将Rails 4.2.4升级到Rails 5.1.0.rc1 API项目

Question

我有一个使用Rails 4.2.4，Mongoid 4的项目，我想升级到Rails 5.1.0.rc1和Mongoid 6.1.0。

我已按照[here]（http://railsapps.github.io/updating-rails.html）的说明操作，但是当我运行命令bundle update时，我收到以下错误：

Bundler could not find compatible versions for gem "rack":
In Gemfile:
rails (= 5.1.0.rc1) was resolved to 5.1.0.rc1, which depends on
  actionpack (= 5.1.0.rc1) was resolved to 5.1.0.rc1, which depends on
    rack (~> 2.0)

sinatra (>= 1.3.0) was resolved to 1.3.0, which depends on
  rack (~> 1.3)

为什么有任何想法？