Use HTTPS in CDN URLs in _config.yml

For consistency with the rest of the docs and per our code guidelines.
Merge pull request #14192 from twbs/https-everywhere

Use HTTPS in CDN URLs in _config.yml
grunt dist
Document that form field feedback icons only work with `input.form-control`

Closes #14202.

[skip sauce]
Merge pull request #14203 from twbs/fix-14202

Document that form field feedback icons only work with input.form-control
update to rouge v1.5.1 in Travis CI build
update to Jekyll v2.1.1 in Travis CI build
Update grunt-csscomb.