Should use `$stdout/$stderr/$stdin` instead of `STDOUT/STDERR/STDIN`

> Use `$stdout/$stderr/$stdin` instead of `STDOUT/STDERR/STDIN`. `STDOUT/STDERR/STDIN` are constants, and while you can actually reassign (possibly to redirect some stream) constants in Ruby, you'll get an interpreter warning if you do so.
> – https://github.com/bbatsov/ruby-style-guide