change note in .resizable() mixin for accuracy

Not a Safari bug. It's standards-compliant.

Quoting from http://www.w3.org/TR/css3-ui/#resize :
> ### 8.1. `resize` property
>
> * Applies to: elements with `overflow` other than `visible`
>
> The `resize` property applies to elements whose computed `overflow` value is something other than `visible`.

Original comment was added in commit 648c4689273647c321dd6e3979d910282e9a9339.
add `target` option to Affix plugin
Merge branch 'impl-13342'; Closes #13342.
add missing event namespacing in modal.js
Merge pull request #13600 from twbs/resize-note-correction

change note in .resizable() mixin for accuracy
update link to Grunt setup & info
add to TRAVIS_COMMIT_MSG, TWBS_DO_VALIDATOR, TWBS_DO_SAUCE vars to Travis
Travis: skip RubyGems install+cache when TWBS_DO_VALIDATOR=0
Gruntfile: extract runSubset() func
Gruntfile: skip steps when $TWBS_DO_*=0
prove that [skip sauce] works

add extra newline to .travis.yml that I'll remove momentarily
prove that [skip validator] works

removes the extra newline added in the previous commit
Merge pull request #13623 from twbs/skip-ci-steps

add [skip sauce] and [skip validator]
Merge pull request #13615 from twbs/namespacing

add missing event namespacing in modal.js
fix typo in assertion description