Tell reprepro not to delete extra debs

Summary:
By default, `reprepro` will delete old versions -- so, for
example, adding 3.4.0 will delete 3.3.1, including the deb. The only
thing that saved us from nuking all these debs from our repo was that we
don't pass `--delete` to `rsync` on release builds -- so even though it
gets removed locally, we don't push that delete upstream.

Since we like to keep around the old debs, let's explicitly pass options
to `reprepro` so that it doesn't delete old debs.

We're going to have to think through this more carefully when we make
the next 3.3 point release -- we'll build and push a deb (which would be
deleted before push without this change), but it won't actually be
listed in the repro, since you only list the latest version, unless we
call the package HHVM-LTS or HHVM33 or something like that.

Test Plan: Read the manpage, read the change carefully.

Reviewers: sgolemon, fredemmott, ptarjan

Reviewed By: ptarjan

Differential Revision: https://phabricator.fb.com/D1683301

Signature: t1:1683301:1416012674:457fb7f6aae7a58a2b54e6926b61626ffa91f7b3