Update devDependencies, gems and lots of cleanup/build fixes.

* switch to grunt-postcss and autoprefixer directly; this is the recommended way.
* uglify: specify `ie8` for compatibility
* specify `cascade:false` for autoprefixer
* specify `ieCompat` for less
* drop grunt-contrib-compress since it doesn't work with Node.js 10.x
* remove grunt-contrib-htmlmin to match the v4 docs
* clean up Gruntfile.js
* clean up .travis.yml and backport changes from v4
* build ie10-viewport-bug-workaround.less since it's used in examples
* move shrinkwrap to root
* bs-commonjs-generator.js: Use `path.posix`
* remove .hound.yml
* remove references to no longer used PR bots
* backport and adapt the sri generation script from v4

README.md

@@ -6,7 +6,6 @@
 [![Build Status](https://img.shields.io/travis/twbs/bootstrap/master.svg)](https://travis-ci.org/twbs/bootstrap)
 [![devDependency Status](https://img.shields.io/david/dev/twbs/bootstrap.svg)](https://david-dm.org/twbs/bootstrap#info=devDependencies)
 [![NuGet](https://img.shields.io/nuget/v/bootstrap.svg)](https://www.nuget.org/packages/Bootstrap)
-[![Selenium Test Status](https://saucelabs.com/browser-matrix/bootstrap.svg)](https://saucelabs.com/u/bootstrap)
 
 Bootstrap is a sleek, intuitive, and powerful front-end framework for faster and easier web development, created by [Mark Otto](https://twitter.com/mdo) and [Jacob Thornton](https://twitter.com/fat), and maintained by the [core team](https://github.com/orgs/twbs/people) with the massive support and involvement of the community.