Detect bottle download strategy

Use ::detect method of DownloadStrategyDetector to detect download strategy of a bottle from its root_url.

This allows bottle root_urls to be s3:// style URLs and be handled by the S3DownloadStrategy without specifying ':using =>'

Library/Homebrew/software_spec.rb

@@ -318,7 +318,7 @@ class Bottle
   end
 
   def select_download_strategy(specs)
-    specs[:using] ||= CurlDownloadStrategy
+    specs[:using] ||= DownloadStrategyDetector.detect(@spec.root_url)
     specs
   end
 end