Gradle DSL method not found: ‘runProguard’

I get an error after updating from my last project. Not a problem in my code but I’m having trouble with build.gradle. How can I fix it?

build.gradle code here:

apply plugin: 'android'

android {
    compileSdkVersion 21
    buildToolsVersion '20.0.0'

    packagingOptions {
        exclude 'META-INF/DEPENDENCIES'
        exclude 'META-INF/LICENSE'
        exclude 'META-INF/LICENSE.txt'
        exclude 'META-INF/license.txt'
        exclude 'META-INF/NOTICE'
        exclude 'META-INF/NOTICE.txt'
        exclude 'META-INF/notice.txt'
        exclude 'META-INF/ASL2.0'
    }

    defaultConfig {
        applicationId 'com.xxx.axxx'
        minSdkVersion 14
        targetSdkVersion 19
        versionCode 6
        versionName '1.0'
    }
    buildTypes {
        release {
            runProguard false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    productFlavors {
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.android.support:appcompat-v7:19.+'
    compile files('libs/commons-codec-1.8.jar')
    compile files('libs/asmack-android-8-4.0.4.jar')
    compile 'com.android.support:support-v4:21.0.0'
    compile 'com.google.code.gson:gson:2.2.4'
    compile 'com.jakewharton:butterknife:5.1.1'
}

Gradle Sync message output:

Error:(27, 0) Gradle DSL method not found: 'runProguard()'
**Possible causes:
The project 'Atomic4Mobile' may be using a version of Gradle that does not contain the method.
**Gradle settings**
The build file may be missing a Gradle plugin.
**Apply Gradle plugin**