Add google() repo

This is required to build the app outside of Android Studio.

Fixes:
 > Could not find com.android.tools.build:gradle:3.0.1.
     Searched in the following locations:
         https://jcenter.bintray.com/com/android/tools/build/gradle/3.0.1/gradle-3.0.1.pom
         https://jcenter.bintray.com/com/android/tools/build/gradle/3.0.1/gradle-3.0.1.jar

build.gradle

@@ -1,6 +1,7 @@
 buildscript {
     repositories {
         jcenter()
+        google()
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:3.0.1'