Why artifact has no classes?

Question

This is my module (Library) build.gradle file and I uploaded my library then when I'm trying to test it using this line compile 'com.abanoub.androidlib:walkthrough:1.0.0:release@aar' the artifact has no classes .. Help please :)

apply plugin: 'com.android.library'

ext {
    bintrayRepo = 'maven'
    bintrayName = 'walkthrough'

    publishedGroupId = 'com.abanoub.androidlib'
    libraryName = 'Walkthrough'
    artifact = 'walkthrough'

    libraryDescription = 'A simple android library to make a simple walkthrough activity'

    siteUrl = 'https://github.com/AbanoubAsaad/Walkthrough'
    gitUrl = 'https://github.com/AbanoubAsaad/Walkthrough.git'

    libraryVersion = '1.0.2'

    developerId = 'abanoubasaad'
    developerName = 'Abanoub Samaan'
    developerEmail = '[e-mail]'

    licenseName = 'The Apache Software License, Version 2.0'
    licenseUrl = 'http://www.apache.org/licenses/LICENSE-2.0.txt'
    allLicenses = ["Apache-2.0"]
}

android {
    compileSdkVersion 23
    buildToolsVersion "23.0.2"
    publishNonDefault true

   defaultConfig {
        minSdkVersion 14
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile 'com.android.support:appcompat-v7:23.1.1'
}
apply from: 'https://raw.githubusercontent.com/nuuneoi/JCenter/master/installv1.gradle'
apply from: 'https://raw.githubusercontent.com/nuuneoi/JCenter/master/bintrayv1.gradle'