Add Kotlin support to app's module.

app/build.gradle

 apply plugin: 'com.android.application'
+apply plugin: 'kotlin-android'
 apply plugin: 'me.tatarka.retrolambda'
 apply plugin: 'com.jakewharton.hugo'
 apply plugin: 'com.github.triplet.play'
@@ -11,6 +12,7 @@ buildscript {
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:2.3.3'
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$rootProject.ext.kotlinVersion"
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files
@@ -92,12 +94,12 @@ dependencies {
 
     compile "com.android.support:appcompat-v7:$rootProject.ext.supportLibraryVersion"
     compile "com.android.support:design:$rootProject.ext.supportLibraryVersion"
-
     compile "com.android.support:support-annotations:$rootProject.ext.supportLibraryVersion"
     compile 'com.android.support.constraint:constraint-layout:1.0.2'
-
     compile 'com.android.support:multidex:1.0.1'
 
+    compile "org.jetbrains.kotlin:kotlin-stdlib-jre7:$rootProject.ext.kotlinVersion"
+
     compile "com.google.firebase:firebase-core:$playLibVersion"
     compile "com.google.firebase:firebase-crash:$playLibVersion"