Update gradle plugin to 3.1.3 and Enable D8

build.gradle

@@ -1,16 +1,13 @@
 buildscript {
     ext.kotlin_version = '1.2.41'
     repositories {
-        maven {
-            // Google Maven Repository
-            url 'https://maven.google.com'
-        }
+        google()
         mavenCentral()
         jcenter()
     }
     dependencies {
         //classpath 'com.android.tools.build:gradle:2.+'
-        classpath 'com.android.tools.build:gradle:3.1.+'
+        classpath 'com.android.tools.build:gradle:3.1.3'
         classpath 'com.google.gms:google-services:3.0.0'
         classpath 'com.github.ksoichiro:gradle-eclipse-aar-plugin:0.3.1'
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
@@ -28,9 +25,7 @@ allprojects {
                 artifact "ivy/[organisation]/[module]/[revision]/[artifact]-[revision].[ext]"
             }
         }
-        maven {
-            url "https://maven.google.com"
-        }
+        google()
         mavenCentral()
         jcenter()
         maven {

gradle.properties

@@ -14,5 +14,7 @@
 # org.gradle.parallel=true
 #Fri Apr 08 18:47:31 EEST 2016
 android.useDeprecatedNdk=true
+
 # for enableD8=true min sdk must be > 22
-android.enableD8=false
+# UPDATE: temporairly commented since gradle plugin updated to 3.1.3 and claims INSTALL_FAILED_DEXOPT is fixed
+# android.enableD8=false