Fix 'branch' in zipdl Lambda code

[robwhelan]

In the zipdl lambda source code, the branch is incorrectly set to a variable named branch_name which is different from other sources.

That means the archive_url is incorrectly constructed for github.

It should read as follows:

branch = event['body-json']['ref'].replace('refs/heads/', '')
archive_url = archive_url.replace('{archive_format}', 'zipball').replace('{/ref}', '/' + branch)