Merge pull request #2232 from alphagov/upgrade-yarn #534

Job	Run time
Integration tests	2m 49s
Lint JavaScript / Run Standardx	14s
Lint Ruby / Run RuboCop	15s
Lint SCSS / Run Stylelint	13s
Security Analysis / Run Brakeman	14s
Test JavaScript / Run Jasmine	43s
Test Ruby / Run RSpec	1m 42s
	6m 10s

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