title: Error:Cannot set readonly property: proguardFiles for class: com.android.build.gradle.managed.BuildType
body: <p>I am trying to run an AR sample app from: <a href="https://artoolkit.org/documentation/doku.php?id=4_Android:android_examples" rel="noreferrer">https://artoolkit.org/documentation/doku.php?id=4_Android:android_examples</a></p>
  
  <p>I tried to open the project ARSimpleProj. But it gives me this error:</p>
  
  <pre><code>Error:Cannot set readonly property: proguardFiles for class: com.android.build.gradle.managed.BuildType
  </code></pre>
  
  <p>I am using Android Studio 2.2.2 and Gradle 2.14.1</p>
  
  <p>Thanks!</p>