Upgrade gradle plugin

lib/android/app/build.gradle

@@ -3,7 +3,7 @@ apply from: '../prepare-robolectric.gradle'
 
 android {
     compileSdkVersion 25
-    buildToolsVersion "27.0.1"
+    buildToolsVersion '27.0.3'
 
     defaultConfig {
         minSdkVersion 19

playground/android/build.gradle

@@ -8,7 +8,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.0.1'
+        classpath 'com.android.tools.build:gradle:3.1.0'
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files