title: Get Spacemacs/Emacs GUI version to recognize nix-shell environment
body: <p>I do my development in a Nix shell (create a default.nix file in my project root and then run <code>nix-shell .</code> to give me a shell with access to the project dependencies).</p>
  
  <p>Spacemacs is my main editor, but when I try to run the GUI version via <code>emacs &amp;</code> I don't have access to the programs in my nix-shell (if I were in a Ruby on Rails project for example, and Ruby was declared as a dependency in my <code>default.nix</code>, I would have no syntax highlighting in Spacemacs because the GUI version of Emacs doesn't see my Nix-shell dependencies).  If I run <code>:!which ruby</code>, it can't even find the <code>which</code> command.</p>
  
  <p>Right now, I'm running spacemacs via <code>emacs -nw</code> and just using it from the console, but I would really like to be able to use the GUI editor and get the full colorschemes available rather than being limited to the ones that look nice in 256 color mode.  It's also quicker for me to switch between the terminal and the editor than between tmux panes or terminal splits to get to my CLI editor.</p>
  
  <pre><code>with import &lt;nixpkgs&gt; {}; {
    cannyFreeRadicalEnv = stdenv.mkDerivation rec {
      name = "rails-project-env";
      version = "0.1";
      src = ./.;
      buildInputs = [
          stdenv
          ruby_2_2_2
          bundler
          zlib
          postgresql94
          sqlite
          zsh
          git
          nodejs-0_12
      ];
    };
  }
  </code></pre>