title: can't generate signed APK from Android studio "Execution failed for task ':packageRelease'"
body: <p>I am using Android Studio 1.2.2 and tried to generate APK with proguard setting. But I can't do it and got below error message. </p>
  
  <p>"Execution failed for task ':packageRelease'.</p>
  
  <blockquote>
    <p>Unable to compute hash of /Users/Documents/projectX/app/build/intermediates/classes-proguard/release/classes.jar "</p>
  </blockquote>
  
  <p>The proguard setting is just simple. </p>
  
  <pre><code>-dontshrink
  -dontoptimize
  -dontusemixedcaseclassnames
  -dontskipnonpubliclibraryclasses
  -dontpreverify
  -verbose
  </code></pre>
  
  <p>And the gradle file is below. </p>
  
  <pre><code>apply plugin: 'com.android.application'
  apply plugin: 'com.google.gms.google-services'
  
  android {
      compileSdkVersion "Google Inc.:Google APIs:22"
      buildToolsVersion "21.1.2"
  
      defaultConfig {
          applicationId "com.bientus.cirque.sansotong"
          minSdkVersion 15
          targetSdkVersion 22
          versionCode 1
          versionName "1.0"
  //        multiDexEnabled true
      }
      buildTypes {
          debug {
              debuggable true
          }
  
          release {
              minifyEnabled true
              //proguardFiles 'proguard-project.txt', 'proguard.cfg'
              //proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-project.txt', 'proguard.cfg'
              proguardFiles 'proguard-project.txt'
          }
      }
  }
  
  dependencies {
          compile fileTree(dir: 'libs', include: ['*.jar'])
          compile 'com.android.support:appcompat-v7:22.0.0'
          compile "com.google.android.gms:play-services:7.5.0"
  //    compile 'com.android.support:multidex:1.0.0'
  }
  
  buildscript {
      repositories {
  //        mavenCentral()
          jcenter()
      }
      dependencies {
          classpath 'com.android.tools.build:gradle:1.2.3'
          classpath 'com.google.gms:google-services:1.3.0-beta1'
      }
  }
  </code></pre>
  
  <p>Can anybody give any idea or thought? 
  Thank you very much!</p>