Update www/castget to latest upstream release

• Previous release was in 2017, long time.
• The release notes just say "new features and various bug fixes". The important change seems to be this: > Behaviour change: The -f/--filter option and the filter > configuration options now match the URLs of enclosures > instead of the filenames castget infers for them
• While here, change to DISTVERSION.

Not taking maintainership, I just spotted this one in passing.

Reported by: portscout