Rollback kotlin update (1.4.0 -> 1.3.72) (firebase#1905)

Author: rlazo

build.gradle

@@ -15,7 +15,7 @@
 import com.google.firebase.gradle.plugins.license.LicenseResolverPlugin
 
 buildscript {
- ext.kotlinVersion = '1.4.0'
+ ext.kotlinVersion = '1.3.72'
  repositories {
  google()
  jcenter()