title: finished with non-zero exit value 2 when I use google login api
body: <p>I want to access the <code>google account</code> to <em>login my app</em>,and I get:</p>
  
  <pre><code>Error:Execution failed for task ':app:dexDebug'.
  &gt; com.android.ide.common.process.ProcessException: org.gradle.process.internal.ExecException: Process 'command '/Library/Java/JavaVirtualMachines/jdk1.8.0_40.jdk/Contents/Home/bin/java'' finished with non-zero exit value 2
  </code></pre>
  
  <p>My <code>build.grade</code>:</p>
  
  <pre><code>    apply plugin: 'com.android.application'
  
      android {
          compileSdkVersion 22
          buildToolsVersion '22.0.1'
  
          defaultConfig {
              applicationId "nthu.nf.project.moocs"
              minSdkVersion 15
              targetSdkVersion 21
              versionCode 3
              versionName "1.2"
          }
          buildTypes {
              release {
                  minifyEnabled false
                  proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
              }
          }
      }
  
      dependencies {
          compile fileTree(dir: 'libs', include: ['*.jar'])
          compile 'com.android.support:appcompat-v7:22.2.0'
          compile 'com.google.android.gms:play-services-identity:7.5.0'
          compile 'com.google.android.gms:play-services-plus:7.5.0'
  
  
      }
  </code></pre>
  
  <p>and in my <code>app/libs</code> I only use:</p>
  
  <pre><code>    YouTubeAndroidPlayerApi.jar
      google-play-services.jar
      commons-io-2.4.jar
  </code></pre>