I have the following bootstrap html code (its JSX hence the className but the idea is the same):
btn-group has display:inline-block so you would use text-center in the parent container..
http://codeply.com/go/hyUYkUrtRN
NOTE: In Bootstrap 4, center-block is now mx-auto, representing margin: 0 auto; for centering display:block elements. Bootstrap 4 now has a d-block class too so an inline element can be made display:block like this..
<img src=".." class="d-block mx-auto" >
Also see: Center the content inside a column in Bootstrap 4
Bootstrap 4 does not have center-block Instead, I use d-block mx-auto which sets display to block, and left and right margins to auto.
Bootstrap 4 (as of 2017-08-16) will use d-block and to center you use mx-auto.