Support older net-sftp API for Ruby 2.0

Old Rubies such as Ruby 2.0 resolve a version of net-sftp that does not
have the `File#size` convenience method. To support these older
versions, use the lower level `File#stat`, which works for versions of
net-sftp all the way back to 2.1.2.

This fixes a `NoMethodError` when SFTP is used via sshkit on Ruby 2.0.

lib/sshkit/backends/netssh/sftp_transfer.rb

@@ -29,7 +29,7 @@ def download!(remote, local, options)
         end
 
         def on_get(download, entry, offset, data)
-          entry.size ||= download.sftp.file.open(entry.remote, &:size)
+          entry.size ||= download.sftp.file.open(entry.remote) { |file| file.stat.size }
           summarizer.call(nil, entry.remote, offset + data.bytesize, entry.size)
         end