Special handling of download url for version 0.7.0 and 0.7.1

This pull request addresses issue #1.  I only handled 0.7.0 and 0.7.1 as I don't know how future versions of packer will publish artifacts.
Change this to use the [`versioncmp`](https://docs.puppetlabs.com/references/latest/function.html#versioncmp) function:
```puppet
if versioncmp($version, '0.7.0') >= 0 {
```
This conditional will apply the prefix to any later Packer versions.