code: private void ensureGroovyComparability(final ClassLoader classLoader) throws MojoExecutionException {
      Version groovyVersion = groovyVersionHelper.detectVersion(classLoader);
      if (groovyVersion == null) {
        // complain and continue
        log.error("Unable to determine Groovy version");
      }
      else {
        log.debug("Detected Groovy version: {}", groovyVersion);
        if (versionHelper.before(2).containsVersion(groovyVersion)) {
          throw new MojoExecutionException("Unsupported Groovy version: " + groovyVersion);
        }
      }
    }
docs: Ensure Groovy compatibility.  Requires Groovy 2+