Update installation.md

To prevent weird Amazon S3 name to be the result of the download. 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\=
Merge pull request #46 from tscheepers/patch-1

Update installation.md to prevent wierd S3 name when downloading