Bundler找不到兼容版本-没有矛盾

Question

我正在尝试bundle update几个宝石，并从捆绑器（版本1.16.2）中获得以下错误（以及其他错误）：

Bundler could not find compatible versions for gem "activemodel":
  In Gemfile:
    rails (~> 4.0.13) was resolved to 4.0.13, which depends on
      activerecord (= 4.0.13) was resolved to 4.0.13, which depends on
        activemodel (= 4.0.13)

    draper (~> 1.0) was resolved to 1.4.0, which depends on
      activemodel (>= 3.0)

    simple_form (~> 3.1.1) was resolved to 3.1.1, which depends on
      activemodel (~> 4.0)

我完全看不到这些要求中的任何矛盾！

Bundler为什么不只使用activemodel版本4.0.13？