title: Phoenix umbrella app code reloader failing after changes of compile.lock and mix.exs
body: <p>I'm experiencing an issue with an umbrella app composed of a phoenix app and proxy app which depends upon <a href="https://github.com/jesseshieh/master_proxy" rel="nofollow noreferrer">master_proxy</a> (which permits to serve different sites with a single port, useful for heroku env for example).</p>
  
  <p>I get this runtime error</p>
  
  <pre><code>** (RuntimeError) could not compile application: proxy.
  
  You must restart your server after changing the following config or lib files:
  
    * redacted_path/umbrella_app_name/_build/dev/lib/proxy/.mix/compile.lock
    * mix.exs
  </code></pre>