Use Amber::Pipe::Static handler to set http response cache headers

Enables caching of static files in `public/` dir which
will improve page load time and prevent delays in applying css/js to the
dom.

https://devcenter.heroku.com/articles/increasing-application-performance-with-http-cache-headers

config/routes.cr

@@ -10,7 +10,7 @@ Amber::Server.instance.config do |app|
 
   # All static content will run these transformations
   pipeline :static do
-    plug HTTP::StaticFileHandler.new("./public")
+    plug Amber::Pipe::Static.new("./public")
     plug HTTP::CompressHandler.new
   end