AO3-6730 Bump rexml from 3.2.6 to 3.2.8 #4080

Job	Run time
rake db:otwseed	1m 11s
rspec spec/controllers	10m 8s
rspec spec/models	5m 29s
rspec --exclude-pattern 'spec/{controllers,models}/*/.rb'	7m 40s
cucumber features/admins	4m 32s
cucumber features/bookmarks	4m 42s
cucumber features/collections	5m 51s
cucumber features/comments_and_kudos	5m 56s
cucumber features/gift_exchanges	6m 23s
cucumber features/importing	2m 40s
cucumber features/other_a	9m 43s
cucumber features/other_b	5m 41s
cucumber features/prompt_memes_a	3m 44s
cucumber features/prompt_memes_b	3m 33s
cucumber features/prompt_memes_c	4m 46s
cucumber features/search	6m 22s
cucumber features/tag_sets	2m 58s
cucumber features/tags_and_wrangling	5m 41s
cucumber features/users	4m 20s
cucumber features/works	12m 43s
	1h 54m 3s

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