Implement remote file check as Haskell command

lib/download.rb

@@ -48,13 +48,7 @@ module SlackBuilder
   end
 
   def self.remote_file_exists?(url)
-    uri = URI hosted_sources(url)
-
-    request = Net::HTTP.new uri.host, uri.port
-    request.use_ssl = true
-    response = request.request_head uri.path
-
-    response.code.to_i == 200
+    `./bin/slackbuilder exists #{url}`.strip == 'True'
   end
 
   def self.download_and_deploy(uri, tarball)