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

.gitignore

@@ -37,5 +37,7 @@ Thumbs.db
 docs/.jekyll-metadata
 
 # Folders to ignore
+/.bundle/
 bower_components
 node_modules
+/vendor/