Error testing spock mandril grails v2.4.3

I'm testing a Grails application but i get this error message:

I'm trying to do unit test. My Grails version is 2.4.3

Error executing script TestApp: java.lang.ClassNotFoundException: grails.plugin.spock.

That's my code:

@TestFor(TrackEmailController)
@Mock(TrackEmail)
class TrackEmailUnit2Spec extends Specification {

    def setup() {
    }

    def cleanup() {
    }

    void "test something"() {
    }

    void 'test mandrillEventsEndPoint'() {
        when:
        controller.mandrillEventsEndPoint()

        then:
        response.text == 'mandrill Events End Point'
    }
}