use S3-cached Jekyll install to further speed up the Travis build

After a \*lot\* of pain, I am pleased to present a further improvement to the build process:
Caching the installation of Jekyll, using S3.

This approximately halves the duration of the HTML validation sub-build:
* [2 min 4 sec](https://travis-ci.org/twbs/bootstrap/jobs/15509333) → [1 min 10 sec](https://travis-ci.org/twbs/bootstrap/jobs/15511489)

X-Ref: #11726.