Update installation.md to prevent wierd S3 name when downloading

To prevent weird Amazon S3 name to be the result of the download. With the current code the ouput file could be called something like `702fe616-4502-11e3-8d9a-b95576a2176f.gz\?response-content-disposition\=attachment\;\ filename\=confd_0.2.0_linux_amd64.tar.gz\&AWSAccessKeyId\=AKIAISTNZFOVBIJMK3TQ\&Expires\=1394028017\&Signature\=3%2FpqUGYkufhsv0Z4SrMn7oQ4sW0\=`