Update node_js versions for Travis
Update node_js versions for Travis

joelanman/govuk_elements,gavboulton/govuk_elements,gavboulton/govuk_elements,gavboulton/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,joelanman/govuk_elements