Revert "Temporarily revert README deletions from #13470,"

This reverts commit 54284a13c6b503d925bf6f75db2830f1af7fcf98.
fix #13386
Always use preserveComment: 'some'
Make autoprefixer browsers fixed

Closes #13795 by merging it.
popovers.html: fix typo: "Dimissmisable"
Fix dismissible spelling
Merge pull request #13845 from BBosman/dismissible

Fix dismissible spelling
fix for #13747

Closes #13782 by merging it.
Merge branch 'erwanjegouzo-master'
grunt dist
Update dependencies.
Gruntfile: add note about duplicate copy of jqueryCheck code

Refs #13842
Merge branch 'deps'
address #13811
Travis: per today's PyPI issues, travis_retry the pip install step

Also, remove deprecated --use-mirrors option
sigh
FFS
Merge pull request #13842 from twbs/fix-13811

update "is jQuery defined" check for UMD compatibility
Run `grunt`.
Add white-space: nowrap; to .dropdown-header

Dropdowns adjust their width to the width of their containing a elements - which have no line breaks ([*](https://github.com/twbs/bootstrap/commit/f226463b057f945795661c3d19fbcabf63b931f7)). Consistent to this dropdown-headers also should be prevent to contain line breaks.