tool/downloader.rb: retry Net::ReadTimeout

as failed in
https://ci.appveyor.com/project/ruby/ruby/builds/19540849/job/9p9pq0b7hfrb2r8q

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65097 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/downloader.rb

@@ -271,7 +271,7 @@ class Downloader
     times = 0
     begin
       block.call
-    rescue Errno::ETIMEDOUT, SocketError, OpenURI::HTTPError => e
+    rescue Errno::ETIMEDOUT, SocketError, OpenURI::HTTPError, Net::ReadTimeout => e
       raise if e.is_a?(OpenURI::HTTPError) && e.message !~ /^50[023] / # retry only 500, 502, 503 for http error
       times += 1
       if times <= max_times