Run the strings:validate:reference during validate

If the documentation is not up-to-date or is broken, we want the
validate task to fail so that we can more quickly detect inconsistencies
and fix them early.

lib/puppetlabs_spec_helper/rake_tasks.rb

@@ -228,6 +228,10 @@
       warn 'Skipping metadata validation; the metadata-json-lint gem was not found'
     end
   end
+
+  if File.exist?('REFERENCE.md')
+    Rake::Task['strings:validate:reference'].invoke
+  end
 end
 
 task :metadata do