Android Studio中沒有ConstraintLayout

我學習如何使用ConstraintLayout構建響應式UI，方法是遵循此article。但是我的Android項目中看不到ConstraintLayout選項。Android Studio中沒有ConstraintLayout

的build.gradle

apply plugin: 'com.android.application' 

android { 
    compileSdkVersion 23 
    buildToolsVersion "23.0.2" 

    defaultConfig { 
     applicationId "com.example.tony.chatapp" 
     minSdkVersion 15 
     targetSdkVersion 23 
     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-v7:23.1.1' 
    compile 'com.android.support.constraint:constraint-layout:1.0.2' 
} 

repositories { 
    maven { 
     url 'https://maven.google.com' 
    } 
}