Load jquery from Google CDN. Backup to hosted version, if CDN fails

index.html

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script>
  if(typeof jQuery=="undefined"){
    document.write(unescape("%3Cscript src='/js/jquery-1.7.1.min.js' type='text/javascript'%3E%3C/script%3E"));
  }
</script>

index.html.erb

<%# In your Ruby: @environment = ENV['RACK_ENV'] %>

<% if @environment == 'development' %>
  <script src="/js/jquery-1.7.1.min.js" type="text/javascript"></script>
<% else %>
  <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
  <script>
    if(typeof jQuery=="undefined"){
      document.write(unescape("%3Cscript src='/js/jquery-1.7.1.min.js' type='text/javascript'%3E%3C/script%3E"));
    }
  </script>
<% end %>

index.html has what you want for static HTML. If you're using a framework, then you could also conditionally use a locally hosted version if you're in development environment. For Sinatra, you can check the RACK_ENV environment variable. See index.html.erb.