Fix ArsModelDownloads which caused Akeeba Live Update to malfunction and likely caused the blank item page issue

Blank item page issue: https://www.akeebabackup.com/support/akeeba-release-system/17087-blank-item-page.html
Fix undefined $size variables. I think this is ok, but please verify.
Merge pull request #7 from jurianeven/fix-ArsModelDownloads

Fixes for ArsModelDownloads