Update ruby-ffi version

[mnightwardskytap]

Hello all,

According to https://nvd.nist.gov/vuln/detail/CVE-2018-1000201 ffi versions 1.9.23 and earlier have a DLL loading issue. Can https://github.com/guard/rb-inotify/blob/master/rb-inotify.gemspec be updated to require v1.9.24 or later?

Thanks!