AffectedModuleDetector: Task 'runAffectedUnitTests' not found in root project

Whenever I apply the plugin to a project and run the ./gradlew runAffectedUnitTests -Paffected_module_detector.enable command, I get the following error:

FAILURE: Build failed with an exception.

* What went wrong:
Task 'runAffectedUnitTests' not found in root project 'my-project'.

Running ./gradle tasks or ./gradlew tasks -Paffected_module_detector.enable results in no tasks added by this plugin either.

I have tried with multiple Android/Java only projects which all have different configurations, but all come up with the same results. I even copy pasted this entire block into the root build.gradle (thus having two buildscript blocks) to no success.

buildscript {
  repositories {
    maven {
      url "https://plugins.gradle.org/m2/"
    }
  }
  dependencies {
    classpath "com.dropbox.affectedmoduledetector:affectedmoduledetector:0.1.0"
  }
}
apply plugin: "com.dropbox.affectedmoduledetector"