Sinatra: Rails 5.0/ Ruby 2.3.0 兼容性

我正在尝试在我的 rails 应用程序中安装最新版本的 sinatra gem（ruby 版本 2.3.0，rails 版本 5.0.0.beta1）。 它在没有指定版本时安装 sinatra-1.0，但是当我指定版本时，比如 1.4.6，它说

Bundler 找不到 gem "rack" 的兼容版本：
在快照（Gemfile.lock）中：
机架（= 2.0.0.alpha）

在 Gemfile 中：
rails (< 5.1, >= 5.0.0.beta1) 被解析为 5.0.0.beta1，这取决于
railties (= 5.0.0.beta1) 被解析为 5.0.0.beta1，这取决于
actionpack (= 5.0.0.beta1) 被解析为 5.0.0.beta1，这取决于
机架 (~> 2.x)

rails (< 5.1, >= 5.0.0.beta1) was resolved to 5.0.0.beta1, which depends on
  railties (= 5.0.0.beta1) was resolved to 5.0.0.beta1, which depends on
    actionpack (= 5.0.0.beta1) was resolved to 5.0.0.beta1, which depends on
      rack-test (~> 0.6.3) was resolved to 0.6.3, which depends on
        rack (>= 1.0)

rails (< 5.1, >= 5.0.0.beta1) was resolved to 5.0.0.beta1, which depends on
  railties (= 5.0.0.beta1) was resolved to 5.0.0.beta1, which depends on
    actionpack (= 5.0.0.beta1) was resolved to 5.0.0.beta1, which depends on
      rack-test (~> 0.6.3) was resolved to 0.6.3, which depends on
        rack (>= 1.0)

sinatra (= 1.4.6) was resolved to 1.4.6, which depends on
  rack (~> 1.4)

rails (< 5.1, >= 5.0.0.beta1) was resolved to 5.0.0.beta1, which depends on
  sprockets-rails (>= 2.0.0) was resolved to 3.0.0, which depends on
    sprockets (>= 3.0.0) was resolved to 3.5.2, which depends on
      rack (< 3, > 1)