Using gradle extra properties to manage Android dependency versioning

app-module-build.gradle

// app module build.gradle
apply plugin: 'com.android.application'

android {
    compileSdkVersion rootProject.compileSdkVersion
    buildToolsVersion rootProject.buildToolsVersion
    defaultConfig {
        applicationId "com.segunfamisa.gradleextraproperties"
        minSdkVersion rootProject.minSdkVersion
        targetSdkVersion rootProject.targetSdkVersion
        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:$rootProject.supportLibraryVersion"
    compile "com.android.support:design:$rootProject.supportLibraryVersion"
    compile "com.android.support:percent:$rootProject.supportLibraryVersion"
    compile "com.android.support:cardview-v7:$rootProject.supportLibraryVersion"
    compile "com.android.support:gridlayout-v7:$rootProject.supportLibraryVersion"

    //play services
    compile "com.google.android.gms:play-services-location:$rootProject.playServicesVersion"
    compile "com.google.android.gms:play-services-gcm:$rootProject.playServicesVersion"

    testCompile "junit:junit:$rootProject.jUnitVersion"
}

top-level-build.gradle

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.2.0-alpha4'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

ext {
    // sdk and tools
    minSdkVersion = 14
    targetSdkVersion = 23
    compileSdkVersion = 23
    buildToolsVersion = '23.0.2'

    supportLibraryVersion = '23.4.0'
    playServicesVersion = '9.0.0'

    jUnitVersion = '4.12'
}