later 1.4.1

• Fixed #203: Resolves an issue where packages that have LinkingTo: later (including promises and httpuv) and were built against later 1.4.0, would fail to load on systems that actually had older versions of later installed, erroring out with the message "function 'execLaterFdNative' not provided by package 'later'". With this fix, such dependent packages should gracefully deal with older versions at load time, and complain with helpful error messages if newer C interfaces (than are available on the installed later) are accessed. (#204)