Mark place-self supported in Safari 11+ (#6847)
Mark place-self supported in Safari 11+ (#6847)

This change marks the CSS place-self property as supported in Safari 11,
per https://developer.apple.com/safari/technology-preview/release-notes/#r31
(TP 31) and https://trac.webkit.org/changeset/216829/webkit (2017-05-13).

Fixes https://github.com/mdn/browser-compat-data/issues/6846
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data