Clean up the Rakefile.

Courtesy of @mschulkind: https://github.com/mschulkind/boxes/commit/f2e0074998f24b8d5589aa59ffdb0572d12bdfa2
Updates the Precise and Wheezy ISO references.
Fixes builds with Packer > 0.5. Closes #7.

Also courtesy of @mschulkind: https://github.com/mschulkind/boxes/commit/f2e0074998f24b8d5589aa59ffdb0572d12bdfa2