Problems with Proguard?

Error:Execution failed for task ':app:proguardRelease'.
> java.io.IOException: Please correct the above warnings first.

apply plugin: 'com.android.application'

android {
    compileSdkVersion 20
    buildToolsVersion "20.0.0"

    defaultConfig {
        applicationId "com.example.kevin.ribbit"
        minSdkVersion 15
        targetSdkVersion 20
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            runProguard true
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile 'com.android.support:support-v13:20.0.0'
    compile files('libs/commons-io-2.4.jar')
    compile files('libs/picasso-2.3.4.jar')
    compile files('libs/Parse-1.5.1.jar')
}

# Add project specific ProGuard rules here.
# By default, the flags in this file are appended to flags specified
# in C:\Users\Kevin\AppData\Local\Android\android-studio\sdk/tools/proguard/proguard-android.txt
# You can edit the include path and order by changing the proguardFiles
# directive in build.gradle.
#
# For more details, see
#   http://developer.android.com/guide/developing/tools/proguard.html

# Add any project specific keep options here:

# If your project uses WebView with JS, uncomment the following
# and specify the fully qualified class name to the JavaScript interface
# class:
#-keepclassmembers class fqcn.of.javascript.interface.for.webview {
#   public *;
#}


-assumenosideeffects class android.util.Log {
    public static boolean isLoggable(java.lang.String, int);
    public static int v(...);
    public static int i(...);
    public static int w(...);
    public static int d(...);
    public static int e(...);
}