title: Error in activeadmin gem
body: <p>hello I am here again with an error in my project.
  I want to install activeadmin gem but it is showing me error as below.my rails version is 4.0.0</p>
  
  <p>I looked to the solution provided but then also it shows me error.</p>
  
  <pre><code>Updating git://github.com/gregbell/active_admin.git
  fatal: ambiguous argument 'rails4': unknown revision or path not in the working
  tree.
  Use '--' to separate paths from revisions, like this:
  'git &lt;command&gt; [&lt;revision&gt;...] -- [&lt;file&gt;...]'
  Git error: command `git rev-parse rails4` in directory
   C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/cache/bundler/git/active_admin-d
   67faab65e9b74efbc8efb4a777a851e9f78b2ca has failed.
  If this error persists you could try removing the cache directory
  'C:/RailsInstaller/Ruby2.0.0/lib/ruby/gems/2.0.0/cache/bundler/git/active_admin-
   d67faab65e9b74efbc8efb4a777a851e9f78b2ca'
  </code></pre>
  
  <p>If I remove the branch name it shows me error</p>
  
  <pre><code>Updating git://github.com/gregbell/active_admin.git
  Resolving dependencies...
  Bundler could not find compatible versions for gem "devise":
   In Gemfile:
  activeadmin (&gt;= 0) x86-mingw32 depends on
    devise (~&gt; 3.2) x86-mingw32
  
  devise (3.1.1)
  </code></pre>