Update scrollspy.js

Scrollspy target in tab content does not work properly. Calling .parents('.active') will return all parents with an active class (including the tab pane). Changing this line to .parentsUntil(this.options.target, '.active') should resolve the issue. This will scope the query to only search for active elements inside the scrollspy target.
Added unit test for removing only active class from within the scroll spy key.
Fix: IE10 or lower incorrectly determines the scrollheight of the body. Use document.documentElement for IE. To make this browser agnostic, the maximum scrollheight of both body and documentElement is used.
Fix: IE10 or lower incorrectly determines the scrollheight of the body. Use document.documentElement for IE. To make this browser agnostic, the maximum scrollheight of both body and documentElement is used.
Fixed flickering and simplified calculations

- Removed offending line causing a flicker to the default top state
- Simplified calculation of the top offset when in the bottom state
Break mixins.less up into smaller modules.

Move all deprecated vendor prefix mixins to less/mixins/vendor-prefixes.less
Create mixins directory and move partials to that directory.

FIXES #12994
Merge branch 'master' into modular-the-mixins

* master:
  Replace some images on the Carousel example with some base64 encoded 1x1 gifs
  ugh grunt
  moar grunt
  mo copy
  http://www.youtube.com/watch?v=AJWtLf4-WWs
  grunt
  Copy change
  fix grammar in comment in forms.less
  Fixes #12759
  Fixes #12738
  replace .visible-{size} with .visible-{size}-{display} ; fixes #8869

Conflicts:
	dist/css/bootstrap-theme.css.map
	dist/css/bootstrap.css.map
	docs/assets/js/customize.min.js
	docs/assets/js/raw-files.min.js
	docs/dist/css/bootstrap-theme.css.map
	docs/dist/css/bootstrap.css.map
	less/mixins.less
Add consistent newline at end of files in less/mixins
Add recompiled assets after nuking toc in mixins.less.
Formatting, comment consolidation, and line endings
Merge branch 'master' into modular-the-mixins

Conflicts:
	dist/css/bootstrap.css.map
	docs/assets/js/raw-files.min.js
	docs/dist/css/bootstrap.css.map
words
Correct the page name

"Getting started" is used in the docs.
Add missing customizer comment
Merge pull request #13002 from ZDroid/hash

Add missing customizer comment
Bump Jekyll version used in Travis to v1.4.2.

Note that while v1.4.3 is the latest stable version, it has a nasty Windows bug.
(https://github.com/jekyll/jekyll/issues/1948)
Merge branch 'master' into modular-the-mixins
even more grunt
Merge pull request #13001 from ZDroid/title

Correct the page name
Merge pull request #12998 from twbs/modular-the-mixins

Modular the mixins