title: debug app build failure after enabling proguard
body: <pre><code>Error:Execution failed for task 
  ':app:transformClassesWithAndroidGradleClassShrinkerForDebug'. 
  Warnings found during shrinking, please use -dontwarn or -ignorewarnings to suppress them.
  </code></pre>
  
  <p>How do i fix this?</p>
  
  <p>PS- also I want to use <code>progaurd</code> in my application.</p>
  
  <pre><code>apply plugin: 'com.android.application'
  
  android {
      compileSdkVersion 26
      buildToolsVersion '26.0.2'
  
      dexOptions{
          javaMaxHeapSize="2g"
      }
  
      defaultConfig {
          applicationId "com.example.projetinnovation.newadvocatediary"
          minSdkVersion 15
          targetSdkVersion 26
          versionCode 1
          versionName "1.0"
          multiDexEnabled=true
  
      }
      buildTypes {
          debug {
              minifyEnabled true
              useProguard false
              proguardFiles getDefaultProguardFile('proguard-andr    oid.txt'),
                      'proguard-rules.pro'
          }
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-android.    txt'), 'proguard-rules.pro'
          }
      }
  }
  
  repositories {
      maven {
          url "https://jitpack.io"
      }
      google()
  }
  dependencies {
      compile fileTree(dir: 'libs', include: ['*.jar'])
      compile project(path: ':agendacalendarview')
      compile 'pl.droidsonroids.gif:android-gif-drawable:1.2.2'
      compile 'com.android.support:appcompat-v7:26.+'
      compile 'com.android.support:design:26.+'
      compile 'com.loopj.android:android-async-http:1.4.9'
      compile 'com.squareup.picasso:picasso:2.5.2'
      compile 'com.mcxiaoke.volley:library:1.0.19'
      compile 'com.google.code.gson:gson:2.8.+'
      compile 'com.github.clans:fab:1.6.4'
      compile 'com.github.dmytrodanylyk.android-process-button:library:1.0.4'
      compile 'com.google.firebase:firebase-messaging:10.0.1'
      testCompile 'junit:junit:4.12'
      compile 'com.github.ybq:Android-SpinKit:1.1.0'
      compile 'com.android.support:cardview-v7:26.+'
      compile 'com.github.d-max:spots-dialog:0.7@aar'
      compile 'com.github.ganfra:material-spinner:2.0.0'
  
      compile 'com.android.support:multidex:1.+'
  }
  
  apply plugin: 'com.google.gms.google-services'
  </code></pre>