Test/Production environment imparity in nokogiri

Test/Production environment imparity in nokogiri exists after !10156 (merged) (2017-05):

Test env: latest (1.8.0 as of 2017-07-11) defined in Gemfile.lock but overridden by scripts/prepare_build.sh (a test log in CI)
(Dev/)Prod env: 1.6.8.1 defined in Gemfile.lock (ref)

Edited Jul 10, 2017 by Takuya Noguchi