Fix exception from out of date ratpack template

The previous script throws an exception when trying to run the Ratpack application.

org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
ratpack.groovy: 1: unable to resolve class org.ratpackframework.groovy.templating.TemplateRenderer
 @ line 1, column 1.
   import org.ratpackframework.groovy.templating.TemplateRenderer
   ^

1 error

	at org.codehaus.groovy.control.ErrorCollector.failIfErrors(ErrorCollector.java:309)