Can't directly use plugin from MavenCentral - Plugin's marker isn't published.

I'm modifying my build to remove the Gradle Plugin Portal from my plugin repositories and the Protobuf plugin is the one giving me an issue. I'm unable to apply the plugin using the below block:

```
plugins {
id "com.google.protobuf" version "0.9.4"
}
```

Instead I needed configure it the old way using buildscript:
```
buildscript {
repositories {
mavenCentral()
}
dependencies {
classpath("com.google.protobuf:protobuf-gradle-plugin:0.9.4")
}
```

It seems like the [plugin's marker artifact](https://docs.gradle.org/current/userguide/plugins.html#sec:plugin_markers) isn't being published to mavenCentral. If that can be fixed then I'd be able to remove the buildscript bit.