* 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`.

follow-up to #11738

update makefile to work again; gitignore the compiled docs _gh_pages folder; test makefile with darkened navbar active bg

Allows you to keep a 'dist' directory in the repository as a target build location.
This can be useful if you have bootstrap as a submodule in a project and want to build it,
but still keep it self-contained. Similar to how jQuery does their default builds:
https://github.com/jquery/jquery/#how-to-build-your-own-jquery

Continous integration is nice, fixes #3155



Signed-off-by: Chris Aniszczyk <zx@twitter.com>

add build script for minified files and allow elements to be returned to getContent and title in popovers