[docs] Upgrade doc generating scripts
[docs] Upgrade doc generating scripts

Docs scripts have dependencies on versions that are affected by CVEs:

- jekyll 3.4.5 is affected by CVE-2018-17567
- ffi 1.9.18 is affected by CVE-2018-1000201

Based on their descriptions neither of these should affect Kudu, but I
think it's still worth upgrading these just in case.

Change-Id: I01cf76c37e26fc12c5b4162c1bcd4262dcd0cacf
Reviewed-on: http://gerrit.cloudera.org:8080/14613
Tested-by: Kudu Jenkins
Reviewed-by: Grant Henke <4cf7ebbe638391c4d27a10cf751b99bdbd1a1880@apache.org>

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