use S3-cached Jekyll install
Merge pull request #11891 from twbs/cache-jekyll-bundle

use S3-cached Jekyll install to further speed up the Travis build
move Sauce test browsers list out of Gruntfile into separate file
Merge pull request #11893 from twbs/separate-browsers-file

move Sauce test browsers list out of Gruntfile into separate file
Merge remote-tracking branch 'upstream/master'
Properly synchronize with upstream