title: Gradle error when adding firebase
body: <p>I have a problem in gradle when I am adding firebase or google ads:</p>
  
  <p><img src="https://i.stack.imgur.com/l4vLx.png" alt="here is the error">. </p>
  
  <p>Also you can check the code from gradle below. What am I missing?</p>
  
  <pre><code>android {
      compileSdkVersion 27
      defaultConfig {
          applicationId "com.jnugzar"
          minSdkVersion 15
          targetSdkVersion 27
          versionCode 1
          versionName "1.0"
  
          testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
      }
      buildTypes {
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
          }
      }
  }
  
  dependencies {
      implementation fileTree(include: ['*.jar'], dir: 'libs')
  
  
      implementation 'com.android.support:appcompat-v7:27.1.1'
      implementation 'com.android.support.constraint:constraint-layout:1.1.2'
      testImplementation 'junit:junit:4.12'
      androidTestImplementation 'com.android.support.test:runner:1.0.2'
  
  
      androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
      implementation 'com.google.firebase:firebase-core:16.0.1'
      implementation 'com.google.firebase:firebase-ads:15.0.1'
  
  
  
  }
  </code></pre>