`PluginDependenciesSpec.kotlin` no longer defaults to `embeddedKotlinVersion`

[bamboo]

With the recent improvements to the plugins DSL it is now possible to use the plugins block in child projects to apply plugins already included on the classpath by the parent. For instance:

// root project
plugins {
  kotlin("jvm") version "1.1.51"
}

// child project
plugins {
  kotlin("jvm")
}

However, plugin requests in child projects must not include a version number which disallows the kotlin("jvm") plugin dependency spec above as it currently always includes a version number.

The sample above would currently fail with:

Plugin request for plugin already on the classpath must not include a version

In order to enable that use-case kotlin("jvm") will no longer imply version embeddedKotlinVersion.

Unfortunately this is a breaking change as the version will now have to be specified at least once at the top most project.

[bamboo]

Resolved in 3a1f2bf.