Fix sample's proguard file. 0.8.1.0

sample/build.gradle

@@ -31,7 +31,7 @@ android {
             signingConfig signingConfigs.production
             minifyEnabled true
             shrinkResources true
-            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'progress-proguard.txt'
+            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
         }
     }
 }