Fix #11513: Don't always float navbar-text

`navbar-text` should only float when the navbar is expanded on larger
screens, otherwise the element might be (hidden) behind `navbar-nav`.
Fixes #11513.
X-Ref: #8820 #8815
Run grunt dist
- Firefox placeholder text is faded out and not the full color. Adding opacity:1 to it returns it to the proper color
- (this is a re-commit since I messed up my branch last time and failed the travis build)
Merge pull request #11526 from erikflowers/firefox-placeholder-fix

Firefox placeholder text is faded out and not the full color.
Merge pull request #11514 from twbs/navbar-text-float-breakpoint

Fix #11513: Don't always float navbar-text
add explanatory comment Re: #11526
Merge branch 'master' of https://github.com/twbs/bootstrap