Update KeyCastr

Some useful info related to the change:

- keycastr/keycastr seems to be the main repo. The version number is newer too.
- keycastr/keycastr links to a download at hosted by lqez/keycastr.
- The former repo at RichardBronosky/keycastr suffers from the minor issue described here: #3947 (except that the clause is now called `postflight`, not `after_install`.*
- "The app must be dragged into the Accessibility list in System Preferences." is a line I would have liked to see while installing. It could have saved me a trip to the issue linked in the Caveats section.

\* I couldn't find documentation for this and had to guess the new clause name. Is it well-documented?