title: All com.google.android.gms libraries must use the exact same version specification
body: <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>