Bump nokogiri from 1.14.3 to 1.15.6 #3939

Job	Run time
rake db:otwseed	1m 17s
rspec spec/controllers	11m 3s
rspec spec/models	5m 32s
rspec --exclude-pattern 'spec/{controllers,models}/*/.rb'	7m 39s
cucumber features/admins	4m 45s
cucumber features/bookmarks	4m 40s
cucumber features/collections	6m 30s
cucumber features/comments_and_kudos	6m 30s
cucumber features/gift_exchanges	6m 49s
cucumber features/importing	2m 41s
cucumber features/other_a	10m 22s
cucumber features/other_b	5m 52s
cucumber features/prompt_memes_a	4m 0s
cucumber features/prompt_memes_b	3m 45s
cucumber features/prompt_memes_c	5m 1s
cucumber features/search	6m 13s
cucumber features/tag_sets	3m 6s
cucumber features/tags_and_wrangling	5m 36s
cucumber features/users	4m 59s
cucumber features/works	12m 47s
	1h 59m 7s

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