title: Build failed while builing in release mode
body: <p>I am trying to generate signed build but it shows the below error:</p>
  
  <pre><code>Warning:there were 87 unresolved references to classes or interfaces.
           You may need to add missing library jars or update their versions.
           If your code works fine without the missing classes, you can suppress
           the warnings with '-dontwarn' options.
           (http://proguard.sourceforge.net/manual/troubleshooting.html#unresolvedclass)
  Warning:there were 2 unresolved references to program class members.
           Your input classes appear to be inconsistent.
           You may need to recompile the code.
           (http://proguard.sourceforge.net/manual/troubleshooting.html#unresolvedprogramclassmember)
  Exception while processing task 
  java.io.IOException: Please correct the above warnings first.
      at proguard.Initializer.execute(Initializer.java:473)
      at proguard.ProGuard.initialize(ProGuard.java:233)
      at proguard.ProGuard.execute(ProGuard.java:98)
      at proguard.gradle.ProGuardTask.proguard(ProGuardTask.java:1074)
      at com.android.build.gradle.tasks.AndroidProGuardTask.doMinification(AndroidProGuardTask.java:139)
      at com.android.build.gradle.tasks.AndroidProGuardTask$1.run(AndroidProGuardTask.java:115)
      at com.android.builder.tasks.Job.runTask(Job.java:48)
      at com.android.build.gradle.tasks.SimpleWorkQueue$EmptyThreadContext.runTask(SimpleWorkQueue.java:41)
      at com.android.builder.tasks.WorkQueue.run(WorkQueue.java:227)
      at java.lang.Thread.run(Thread.java:745)
  :app:dexRelease
  :app:crashlyticsStoreDeobsRelease
  :app:crashlyticsUploadDeobsRelease
  :app:validateExternalOverrideSigning
  :app:packageRelease FAILED
  Error:Execution failed for task ':app:packageRelease'.
  &gt; Unable to compute hash of D:\Project\AppLocker\app\build\intermediates\classes-proguard\release\classes.jar
  Information:BUILD FAILED
  Information:Total time: 5.629 secs
  Information:1 error
  Information:62 warnings
  Information:See complete output in console
  </code></pre>
  
  <p>I tried rebuild/clean build option but still I am getting the same error everytime.</p>
  
  <p>My project has following dependencies:</p>
  
  <pre><code>compile fileTree(include: ['*.jar'], dir: 'libs')
      testCompile 'junit:junit:4.12'
      compile 'com.android.support:appcompat-v7:23.1.1'
      compile 'com.android.support:design:23.1.1'
      compile 'com.android.support:support-v4:23.1.1'
      compile 'com.android.support:preference-v7:23.1.1'
      compile 'com.android.support:preference-v14:23.1.1'
      compile 'com.android.support:recyclerview-v7:23.1.1'
      compile 'com.android.support:cardview-v7:23.1.1'
      compile 'com.android.support:palette-v7:23.1.1'
      compile 'com.cocosw:bottomsheet:1.+@aar'
      compile 'com.github.paolorotolo:appintro:3.2.+'
      compile 'com.pnikosis:materialish-progress:1.7'
      compile 'com.google.android.gms:play-services-ads:8.4.0'
      compile('com.crashlytics.sdk.android:crashlytics:2.5.5@aar') {
          transitive = true;
      }
  </code></pre>
  
  <p>Please suggest how to fix the above issue. Thanks.</p>