Fix the ask to open in browser dialog on Mac.
Fix the ask to open in browser dialog on Mac.

Why:

* GitReflow::Sandbox#ask_to_open_in_browser method currently only works
  for linux.

This change addresses the need by:

* Add OS.mac? to the initial if statement guarding other platforms.

codenamev/gitreflow,reenhanced/gitreflow,codenamev/gitreflow,reenhanced/gitreflow