How it is correct to use

kotlinVersion

in the section

plugins

?

val kotlinVersion: String by extra

plugins {
    kotlin("jvm") version kotlinVersion
}

Unresolved reference: kotlinVersion

Also, you can check official doc about plugins dsl limitations: https://docs.gradle.org/current/userguide/plugins.html#plugins_dsl_limitations

You can't use variables in

plugins

block. should be

plugins {
    kotlin("jvm") version "1.1.60"
}

This shouldn't bee too much trouble, IMHO, because that is probably the only place where you'll have to use version, the rest will follow suit and use the version defined for the plugin, e.g.

dependencies {
    compile("org.jetbrains.kotlin:kotlin-stdlib-jre8")
}