Fix Gradle file in README example

[rdbatch]

Applying frontend plugin before kotlin2js results in an error from Gradle:

> Failed to apply plugin [id 'org.jetbrains.kotlin.frontend']
   > Extension with name 'kotlin' does not exist. Currently registered extension names: [ext, defaultArtifacts, reporting, sourceSets, java]

This can be confusing for people who are looking at the sample code.