update unar url

[eliranmal]

unar url is broken - update it to a working url

[gileswells]

I was going to do this... @xdissent can we get a merge for this kind man?

Actually just tested and it doesn't download as expected.

[thom4parisot]

I would merge #333 instead, to fetch the utility from an official source instead of a possibly dodgy repo.

[eliranmal]

@oncletom seems reasonable, closing this