Fix for OS X USR1 signal
Fix for OS X USR1 signal

It seems that signals code for Linux and OS X (FreeBSD) are different. I
rely on `syscall.SIGUSR1` which should be cross-compatible.

Tested with `kill -usr1 <pid>` on OS X 10.10

kevinrodbe/webhook,moorereason/webhook,imZack/webhook,appneta/webhook,pavlo/webhook,adnanh/webhook,ciarand/webhook,Sanji-IO/webhook