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]