Update `audio/flacon' to version 4.0.0, which features improved visual
style (including new icons), updated translations, and few other fixes.
Alas, this release also marks the drop of Qt4 support.

Reported by: portscout

Because GitHub releases (tarballs) are not fetched with correct modification
time, set TIMESTAMP to 1513612271 which corresponds to commit c9bd11f tagged
as this release.