title: MSBuild framework after updating Xamarin
body: <p>After updating Xamarin I'm getting this error while building:</p>
  
  <pre><code>Construcción fallida. Runtime 'Mono 3.2.5' does not have the MSBuild '4.0' framework installed
  </code></pre>
  
  <p>I get this other if I try to open a xml layout:</p>
  
  <pre><code>System.IO.FileNotFoundException: Could not load file or assembly 'System.Xaml, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies.
  File name: 'System.Xaml, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'
  </code></pre>
  
  <p>I cannot find anything in Google but this unresolved item:</p>
  
  <p><a href="https://bugzilla.xamarin.com/show_bug.cgi?id=17449" rel="nofollow">https://bugzilla.xamarin.com/show_bug.cgi?id=17449</a></p>
  
  <p>Any idea? My Os is Mac Os and it's an Android project.</p>