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

Created by: cvrebert

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 → 1 min 10 sec

X-Ref: #11726 (closed).