Add white-space: nowrap; to .dropdown-header

Dropdowns adjust their width to the width of their containing a elements - which have no line breaks ([*](https://github.com/twbs/bootstrap/commit/f226463b057f945795661c3d19fbcabf63b931f7)). Consistent to this dropdown-headers also should be prevent to contain line breaks.
Darken @gray-light to meet accessibility guidelines

`@gray-light` darkens from #999 to #777, which puts `.text-muted` at the
threshold for the 4.5:1 WCAG minimum contrast[1]. #777:#fff is 4.48:1.

The “Example” headers in docs become #959595, which is contrast ratio
3:1, the minimum for larger text. Since the headers are less important
than the surrounding text, 3:1 is fine and an improvement on the
previous #bbb:#fff (1.92:1).

Fixes issue #13847.

[1] http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
Merge pull request #13854 from ssorallen/gray-light-higher-contrast

Darken @gray-light to meet accessibility guidelines
grunt dist
fixes #10073: add width: 100%; for ie10 and below to size svg.img-responsive correctly
grunt
fixes #13020: proper color for small/.small in list group headings
fixes #13093: reset the background color and remove background image on disabled theme buttons
fix part of #13111: hide the docs nav on small and below devices
Merge pull request #13851 from ahoereth/patch-1

Add white-space: nowrap; to .dropdown-header
Merge pull request #13857 from twbs/list_group_small

List group headings and small/.small
change skew(x,y) to skewX skewY because mdn told me so
fixes #13346: add a variable for .dl-horizontal's offset
fixes #13347: make striped progress bars its own class
Fixes #13400, #13415: actually document the existing practice for links in panel headings
grunt
Merge pull request #13860 from twbs/fixes_13346

Add a variable for .dl-horizontal's offset
Merge branch 'master' of github.com:twbs/bootstrap
grunt
Merge pull request #13859 from twbs/skew_you

Change skew(x,y) to skewX skewY