Delete the old binary zip before downloading the new binary zips.

[DolceTriade]

aria2c doesn't properly truncate the newly downloaded file if the new file is smaller than the old file. So just delete to ensure it is always properly downloaded.

Fixes #102

[slipher]

OK, so we need it just for that file because it's the only one that is

1. downloaded directly (as opposed to unpacked from a zip)
2. does not have a version number in its name
3. is needed by the user

LGTM