try throttling html-proofer to avoid 429s

Author: mlinksva

Rakefile

@@ -4,5 +4,7 @@ require 'html-proofer'
 
 task :test do
   sh 'bundle exec jekyll build'
-  HTMLProofer.check_directory('./_site/', check_html: true).run
+  HTMLProofer.check_directory('./_site/',
+                              check_html: true,
+                              hydra: { max_concurrency: 10 }).run
 end