use `xvfb` to run tests on travis
use `xvfb` to run tests on travis

bitzesty/trade-tariff-frontend,bitzesty/trade-tariff-frontend,alphagov/trade-tariff-frontend,alphagov/trade-tariff-frontend,bitzesty/trade-tariff-frontend,bitzesty/trade-tariff-frontend,alphagov/trade-tariff-frontend,alphagov/trade-tariff-frontend