How to show vi mode in bash?

I'm on Yosemite, using vi mode in bash (set -o vi in .bash_profile) and I want to have the prompt indicate if I'm in edit or command mode. I looked around and found that this feature has been implemented in bash 4.3 (see http://ift.tt/1x5LvhU) and can be turned on by setting the variable show-mode-in-prompt to on. I installed bash 4.3 using homebrew following these instructions. Now, when I execute bash --version I see GNU bash, version 4.3.33(1)-release (x86_64-apple-darwin14.1.0) and brew info readline returns readline: stable 6.3.8 (bottled) so it looks like I'm using the correct versions. However, when I execute set show-mode-in-prompt on or add it to ~/.inputrc based on the readline manual, the prompt still doesn't change based on the vi mode. What am I doing wrong?