KUDU-1883. Support Ruby 2.4 in make docs task
KUDU-1883. Support Ruby 2.4 in make docs task

Bumps the json gem to 1.8.5 in order to support ruby 2.4.
Additionally bumps the libv8 gem to 3.16.14.14 to support xcode 7.3+.

This is the minimal version bump required to fix my issues,
though newer versions of the gems are available.

Change-Id: Ib3da3c0da24a8a51b4ee61dab371fea685717c82
Reviewed-on: http://gerrit.cloudera.org:8080/6022
Tested-by: Kudu Jenkins
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>

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