* Separate configs for libsass and sass.
* Sass compiler selected based on `process.env.TWBS_SASS`.
* Travis:
  * Use Gemfile to manage ruby dependencies.
  * Run core tests with both Sass compilers.
  * Only install/cache ruby gems required by the test subset.
* Grunt: `update-gemfile-lock` task a la `update-shrinkwrap`.

Conflicts:
	docs/components/helpers.md
	scss/_utilities.scss

use :disabled pseudo-class instead of [disabled] attribute

Re-add Firefox placeholder opacity workaround

Remove .hidden and .show classes

Remove min--moz-device-pixel-ratio

bump min iOS in Autoprefixer config to v7

Migration guide catch up

Bootstrap v4 no longer supports the old versions of Firefox affected by this typo bug.

:disabled is supported in IE9+ and all modern browsers

variables: fix fade-in