* 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`.
Instead of our homegrown grunt/shrinkwrap.js
Fixes#13483.
New npm-shrinkwrap.json is human-readable but still in a consistent/stable format.
Should aid debugging, produce better diffs, and be less prone to merge conflicts
