The .travis.yml does this but putting it in the gemspec means it happens
locally as well when running 'rake build'
If this gem, when built does not include govuk-frontend, then it
functionally does not work. Add a check in the gemspec that a file from
govuk-frontend exists, so that building the gem will fail if 'npm
install' has not been run, or if govuk-frontend has been removed.
...
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Done as pair with Jon Heslop, no review required