Rubocop-daemon stalls on macOS when Homebrew netcat is installed

[nc-srstkac]

The rubocop-daemon script is currently setting $NETCAT_CMD to nc on macOS. When netcat from Homebrew is installed, and the $PATH is set up such that /usr/local/bin comes before /usr/bin, rubocop-daemon hangs while trying to run /usr/local/bin/nc.

If we hard-coded $NETCAT_CMD to /usr/bin/nc on macOS, this would probably be fixed.

[adipasquale]

thanks for the fix, I've submitted a PR!