🐛 Fix dashing install

Replaced open with URI.open, as Kernel#open was deprecated in Ruby 2.7
and doens't work anymore with Ruby 3

Details: ruby/open-uri@53862fa

lib/dashing/downloader.rb

@@ -11,7 +11,7 @@ def get_gist(gist_id)
  end
 
  def get_json(url)
- response = open(url).read
+ response = URI.open(url).read
  JSON.parse(response)
  end
  end