Update to Packer 0.7.5 and update version checking

- Bump the default version to the latest (0.7.5)
- Fix the construction of the download link to reflect the new link structure. This may break older versions.
- Previously packer has responded to `packer -v`, this now doesn't work and requires either `packer version` or `packer --version` - the version check is updated accordingly to avoid re-installation on every run.