Vertical Buttongroups should use only class "btn-group-vertical"

[DerFox4]

Hey thank you for the awesome package, we're useing it at work and had found this little issue:

In the current version the vertical buttongroups aren’t displayed correctly.

As in this picture to seen the margin and the edges aren't correct displayed. At the moment elm-bootstrap uses the classes btn-group and btn-group-vertical for vertical buttongroups, but Bootstrap does only use btn-group-vertical.

https://getbootstrap.com/docs/4.3/components/button-group/#vertical-variation