more -i is not portable

ronn has:
```ruby
pager = ENV['MANPAGER'] || ENV['PAGER'] || 'more -is'
```

However, `more -i` does not work on my Debian buster system:
```shell
$ echo | more -is
more: unknown option -is
Try 'more --help' for more information.
```

And therefore, `ronn -m` fails out of the box.