Unison: Use HTTPS, not HTTP

This commit changes the URLs in
`pkgs/applications/networking/sync/unison/default.nix` to use HTTPS,
rather than HTTP.

These URLs refer to the following domains:

  - `www.seas.upenn.edu`, which uses a HTTPS certificate issued by
    “COMODO Extended Validation Secure Server CA”, and
  - `www.cis.upenn.edu`, which uses a HTTPS certificate issued by
    “InCommon RSA Server CA”.