markdownToHtml seems to be failing

Gradle 4.6 here. My build.gradle specifies:

plugins {
    id 'com.github.johnrengelman.shadow' version '4.0.4'
    id 'java'
    id 'edu.sc.seis.launch4j' version '2.4.6'
    id 'org.kordamp.gradle.markdown' version '2.0.0'
}

repositories {
    mavenLocal()
    mavenCentral()
    jcenter()
}

dependencies {
    compile(
        'ch.qos.logback:logback-classic:1.2.3'
        ... (all of my dependencies here)
    )
}

... rest of my build.gradle here

When I run:

./gradlew clean markdownToHtml

I get:

$ ./gradlew clean markdownToHtml

FAILURE: Build failed with an exception.

* Where:
Build file '/Users/myuser/workspace/myapp/build.gradle' line: 14

* What went wrong:
An exception occurred applying plugin request [id: 'org.kordamp.gradle.markdown', version: '2.0.0']
> Failed to apply plugin [id 'org.kordamp.gradle.markdown']
   > Could not find method register() for arguments [markdownToHtml, class org.kordamp.gradle.plugin.markdown.tasks.MarkdownToHtmlTask, org.kordamp.gradle.plugin.markdown.MarkdownPlugin$1@3b46a8ba] on task set of type org.gradle.api.internal.tasks.DefaultTaskContainer.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

kordamp / markdown-gradle-plugin

markdownToHtml seems to be failing #28