Fix bump `@popperjs` from 2.11.5 to 2.11.6

IMO something's missing in the update dependencies commit https://github.com/twbs/bootstrap/commit/db3490788775ac88cc8dcc32f9b8bcd66f95859e about @popperjs.

Here is a proposal to complete this update.

/cc @mdo or @XhmikosR