Update gradle plugin version to avoid error in release build.

Now it needs gradle >= 4.4

android/build.gradle

@@ -3,11 +3,21 @@ buildscript
     repositories
     {
         jcenter()
+        google()
     }
 
     dependencies
     {
-        classpath 'com.android.tools.build:gradle:2.3.0'
+        classpath 'com.android.tools.build:gradle:3.1.1'
+    }
+}
+
+allprojects
+{
+    repositories
+    {
+        jcenter()
+        google()
     }
 }