(<project>/build.gradle):
buildscript {
  dependencies {
    // Add this line
    classpath 'com.google.gms:google-services:4.0.0'
  }
}
(<project>/<app-module>/build.gradle):
dependencies {
  // Add this line
  compile 'com.google.firebase:firebase-core:16.0.0'
}
...
// Add to the bottom of the file
apply plugin: 'com.google.gms.google-services'