Release v5.1.0

Update NEWS file.

[ci skip]

NEWS

 master:
 
+* Add default `content_type_detector` to `UploadedFileAdapter` (#2270)
+* Default S3 protocol to empty string (#2038)
 * Don't write original file if it wasn't reprocessed (#1993)
 * Disallow trailing newlines in regular expressions (#2266)
 * Support for readbyte in Paperclip attachments (#2034)

lib/paperclip/version.rb

 module Paperclip
   unless defined?(Paperclip::VERSION)
-    VERSION = "5.0.0".freeze
+    VERSION = "5.1.0".freeze
   end
 end