* 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`.

Refs commit 6cfd811f.

Travis CI now does this automatically.

[skip sauce]

Also, remove deprecated --use-mirrors option

removes the extra newline added in the previous commit

add extra newline to .travis.yml that I'll remove momentarily

The 1.4.3 Windows bug is fixed, plus the encoding workaround isn't needed anymore.

Note that while v1.4.3 is the latest stable version, it has a nasty Windows bug.
(https://github.com/jekyll/jekyll/issues/1948)

Running "browserstack_runner" task
Using config: /home/travis/build/twbs/bootstrap/browserstack.json
Launching server on port: 8888
Downloading tunnel jar to `/home/travis/build/twbs/bootstrap/node_modules/browserstack-runner/lib/BrowserStackTunnel.jar`
Launching tunnel
Error: Invalid or corrupt jarfile /home/travis/build/twbs/bootstrap/node_modules/browserstack-runner/lib/BrowserStackTunnel.jar
[Runner alert] Tunnel launch timeout
Exiting
Non existent tunnel

Per https://github.com/twbs/bootstrap/issues/11087#issuecomment-31922999
and internal twbs team discussion.
Effectively reverts the deletions from #11701.