Update uri scheme for versions of packer >= 0.7.0 take two

I went to install packer 0.7.2 and noticed that the curl command is failing.

The packer zip files are hosted with a prefix of `packer_` now, it would appear. See the listing at https://dl.bintray.com/mitchellh/packer/

(sorry about the last pull request. tried to overwrite my username/email in the commit history and ended up rewriting the whole history)