All com.google.android.gms libraries must use the exact same version specification
<p>I get the following error message in my build.grade for the line compile 'com.google.android.gms:play-services-gcm:9.4.0'.</p>

<pre><code>All com.google.android.gms libraries must use the exact same version specification (mixing versions can lead to runtime crashes). Found versions 9.4.0, 9.2.1. Examples include com.google.android.gms:play-services-basement:9.4.0 and com.google.android.gms:play-services-ads:9.2.1
</code></pre>

<p><strong>my app build.gradle</strong></p>

<pre><code>apply plugin: 'com.android.application'

android {
    compileSdkVersion 23
    buildToolsVersion "23.0.3"

    defaultConfig {
        applicationId "net.myApp"
        minSdkVersion 14
        targetSdkVersion 21

        // Enabling multidex support.
        multiDexEnabled true
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
        }
    }
}

dependencies {
    compile project(':facebookSDK')
    compile 'com.jpardogo.materialtabstrip:library:1.1.0'
    compile 'com.android.support:support-v4:23.4.0'
    compile 'joda-time:joda-time:2.1'
    compile 'com.google.android.gms:play-services-gcm:9.4.0'
    compile 'com.android.support:multidex:1.0.1'
    compile files('libs/twitter4j-core-4.0.2.jar')

    android {
        useLibrary 'org.apache.http.legacy'
    }

}
</code></pre>

<p><strong>Project-level gradle</strong></p>

<pre><code>buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.1.0'
    }
}

allprojects {
    repositories {
        jcenter()
    }
}
</code></pre>

<p>Could you tell me how to solve this problem?</p>