Do not emit a comma for the final item in JSON output

This avoids producing JSON that is not parsable.

lib/wayback_machine_downloader.rb

@@ -177,9 +177,10 @@ def list_files
  files = get_file_list_by_timestamp
  $stdout = @orig_stdout
  puts "["
- files.each do |file|
+ files[0...-1].each do |file|
  puts file.to_json + ","
  end
+ puts files[-1].to_json
  puts "]"
  end