Add OS-dependent prereq on Mac::FSEvents

[preaction]

https://metacpan.org/pod/Dist::Zilla::Plugin::OSPrereqs lets us define prereqs on a per-OS basis. Automatically install the right things on Mac OS X for the magic rebuild feature of the daemon.

[preaction]

This is not a pure-Perl dependency, so perhaps we need to relax that restriction. It's not a "prereq" per se, we could work without it, and indeed have checks for that, so perhaps the #2 ticket needs to be updated to be a "minimum functionality is pure-Perl".