ZEPPELIN-1554: Upgrade jekyll

[epugh]

What is this PR for?

I wanted to generate the docs site on my Mac, and found a bunch of old dependencies that needed updating. I don't know if this is related to ZEPPELIN-1554, which looks like there was some good work that is similar to what I've done.

If you want, I could pull in some of the other fixes done in https://github.com/apache/zeppelin/pull/1577 in other PR's...

First time? Check out the contributing guide - https://zeppelin.apache.org/contribution/contributions.html

What type of PR is it?

Bug Fix - I think this is right since we need updates to run Jekyll.

Todos

• [ ] - Task

What is the Jira issue?

https://issues.apache.org/jira/browse/ZEPPELIN-1554

How should this be tested?

• Strongly recommended: add automated unit tests for any new or changed behavior
• Outline any manual steps to test the PR here.
• I ran the bundle exec jekyll serve --watch command.

Screenshots (if appropriate)

Questions:

• Does the licenses files need update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? No

[zjffdu]

@epugh Sorry, I forgot to mention that you can check the README.md for how to generate docs. Building docs via docker would be very easy.