Add back ruby/bundler folders so that they don't show up when we switch branches.

This can be removed when v4 hits EOL, along with the old Jekyll docs dist folder.

Add the old Jekyll docs dist folder so that when we switch branches, it doesn't show up.

No more Ruby dependency and most important more than 10 times less build time (at least on Windows)!

With the current docs directory setup, I'm making too many mistakes and have to manually address path changes and directory moves on deploy. This makes for a frustrating experience developing locally and shipping releases. With this PR, we're basically back to the same setup from v3—duplicating the dist directory into our docs directory. Not the most ideal, but very straightforward for me as the release manager.

`$npm_config_tmp` is a non-existent variable.
Also, use double quotes since single ones don't work.
Finally, git ignore the temp file.

[ci skip]

[skip sauce]

Ensure this directory is ignored so that any installed resources from
Bundler aren't handled by Git.

They are grunt-html-validation leftovers.

[ci skip]

* 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>