Fix for `number_to_letter` when mathn has been required

The `number_to_letter` method expects `1 / 26 == 0`, which isn't true if the `mathn` library has been included.

See it in action, open `irb -r roo`:

```ruby
Roo::Base.number_to_letter(27)
# => "AA"
require "mathn"
Roo::Base.number_to_letter(27)
# never finishes executing
```

This is because including `mathn` [duck punches the divide operator](https://github.com/ruby/ruby/blob/trunk/lib/mathn.rb#L11-L17) into returning a `Rational` instead of an integer,  ensuring [the while loop in this method](https://github.com/Empact/roo/blob/master/lib/roo/base.rb#L53-L57) never ends.