Version conflict updating to 8.4.0

Use these dependencies for the project build.gradle

dependencies {
    classpath 'com.android.tools.build:gradle:2.0.0-alpha3'
    classpath 'com.google.gms:google-services:2.0.0-alpha3'
}

and put this at the end of the app-level build.gradle file (after the dependencies).

apply plugin: 'com.google.gms.google-services'

I have no clue why putting this at the end (and not at the beginning ) solves the error.

EDIT 5/1/2016

Ok… So trying to put an end to all problems you guys have faced with my solution

This is my final app level gradle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 23
    buildToolsVersion "23.0.2"

    defaultConfig {
        applicationId "your-app-name"
        minSdkVersion 16
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

repositories {
    jcenter()
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    testCompile 'junit:junit:4.12'
    compile 'com.android.support:appcompat-v7:23.1.1'
    compile 'com.google.android.gms:play-services:8.4.0'
    compile 'com.android.support:design:23.1.1'
    compile 'com.mcxiaoke.volley:library:1.0.6@aar'
}

apply plugin: 'com.google.gms.google-services'

and this is my final project level 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:2.0.0-alpha3'
        classpath 'com.google.gms:google-services:2.0.0-alpha3'

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

allprojects {
    repositories {
        jcenter()
    }
}

Compare this with your own gradle files, and add or modify any values which are different from what I’ve written.