gwenhywfar: 4.17.0 -> 4.20.0

Upstream changes in 4.20.0:

  * Update bank information files for German banks because the format of
    ZKA files has changed in the meantime.

Upstream release notes in German:

https://www.aquamaniac.de/sites/download/releasenote.php?package=01&release=208

Upstream changes in 4.19.0:

  * Add frontend for GTK3.

Upstream release notes in German:

https://www.aquamaniac.de/sites/download/releasenote.php?package=01&release=207

Upstream changes in 4.18.0:

  * Show hashes for MD5, SHA1 and SHA512 in SSL certificates because
    some bank servers display different hashing variants.
  * Let GNUTLS handle the localisation of TrustStore certificates.

Upstream release notes in German:

https://www.aquamaniac.de/sites/download/releasenote.php?package=01&release=206



The updater script (update.sh) actually didn't get the right file to
download, so I temporarily modified it to download file number 2 because
file number 1 is the GPG signature. The reason why I didn't fix the
updater is because the upstream site will have different URLs in the
next upcoming version, so our updater is basically end-of-life for now.

I also changed the package expression to use the version from
sources.nix instead of a hardcoded value.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @goibhniu