I keep getting

Plugin [id: 'org.jetbrains.kotlin.jvm', version: '1.3.0-rc-57'] was not found

and don't see a whole bunch of docs for specifying the kotlin plugin version via buildscript

You may need to add the

kotlinEap()

repository. It's an extension function provided by Gradle, but you can't use it everywhere, so if it doesn't work, try

maven(url = "<https://dl.bintray.com/kotlin/kotlin-eap/>")

instead (basically what the extension function does)