Fixed bug where downloaded Drupal archive was never deleted.

Created in reference to Issue #1022, this PR fixes a bug where the Drupal archive is never marked as a temporary file, and therefore never deleted. If you look at the Drush 6 version of drush_download_file a drush_get_option('cache') check is made, so I'm not sure why Drush 7 removed it in the first place. 

Git reveals:
```
290e0dd9 includes/drush.inc (Jonathan Araña Cruz 2014-11-17 08:23:07 +0000  739)   if ($cache_duration !== 0 && $cache_file = drush_download_file_name($url)) {
```
```
commit 290e0dd94b02584c92611d500ce38578bc26e028
Author: Jonathan Araña Cruz <jonhattan@faita.net>
Date:   Mon Nov 17 08:23:07 2014 +0000

    Cache release xml by default.

```
So I'm not sure if this PR will affect @jonhattan's previous commit. A better solution is welcome.