无法找到 com.google.firebase:firebase-core:9.0.0。

这个问题已经有答案了:

找不到com.google.firebase:firebase-core:9.0.0

在遵循这里和这里有些不一致的指导后，当我进行gradle同步时会出现错误，提示找不到com.google.firebase:firebase-core:9.0.0

有什么想法是为什么会发生这种情况吗？

app/build.gradle

apply plugin: 'com.android.application'
android {
    compileSdkVersion 23
    buildToolsVersion "23.0.3"
    defaultConfig {
        minSdkVersion 15
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    productFlavors {
        free {
            applicationId ... 
        }
        paid {
            applicationId ... 
        }
    }
}
dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    testCompile 'junit:junit:4.12'
    compile 'com.android.support:appcompat-v7:23.3.0'
    compile 'com.android.support:support-v4:23.3.0'
    freeCompile 'com.google.firebase:firebase-core:9.0.0'
    freeCompile 'com.google.firebase:firebase-ads:9.0.0:9.0.0'
}
apply plugin: 'com.google.gms.google-services'

root/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:2.1.0'
        classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'
        classpath 'com.google.gms:google-services:3.0.0'
        // 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
}