I have found lots of posts about this question ,but non of them worked for me.Here is my Gradle:
apply plugin: 'com.android.application'
android {
    compileSdkVersion 23
    buildToolsVersion "23.0.0"
    defaultConfig {
        applicationId "*********"
        minSdkVersion 16
        targetSdkVersion 23
        versionCode 5
        versionName "1.1"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}
dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.android.support:appcompat-v7:23.0.0'
    compile 'com.android.support:cardview-v7:21.0.+'
    compile 'com.android.support:recyclerview-v7:21.0.+'
    compile files('libs/gradle-wrapper.jar')
    compile 'com.facebook.fresco:fresco:0.9.0+'
    compile 'com.facebook.android:audience-network-sdk:4.+'
    compile 'com.google.android.gms:play-services:7.8.0'
}
I can't figure out where there the problem is.