Fix BigDecimal.new deprecation warning
Fix BigDecimal.new deprecation warning

When using Dugway on Ruby 2.6.0, the following deprecation warning is
thrown:

```
warning: BigDecimal.new is deprecated; use BigDecimal() method instead.
```

This reworks the use of `BigDecimal` based on this deprecation warning.

Fixes https://github.com/bigcartel/dugway/issues/170

bigcartel/dugway,bigcartel/dugway,bigcartel/dugway