Change first-child to first-of-type in btn-group

[mermop]

Hello friends! Long-time listener, first-time caller 📞

This PR changes the selector for resetting rounded corners of btn elements in btn-groups from first-child to first-of-type to accommodate hidden inputs in sets of checkboxes or radio buttons.

The Rails collection_check_box or Simple Form or Formtastic all add a hidden input either at the beginning or end of a collection of check boxes or radio buttons. This makes implementing groups of checkboxes or radio buttons as per the documentation either painful or slightly ugly.

Hope this is the right place to put this PR!

Markup using Rails collection_check_boxes

Screenshot using first-child/last-child

Screenshot using first-of-type/last-of-type

[mermop]

Realised this is the wrong place to open this PR - I put it in the bootstrap repo instead