Scheduled weekly dependency update for week 04 (#4251)
Scheduled weekly dependency update for week 04 (#4251)

* Update certifi from 2017.11.5 to 2018.1.18

* Update pytest-variables from 1.7.0 to 1.7.1

* Update pytest-xdist from 1.21.0 to 1.22.0

kumar303/addons-frontend,kumar303/addons-frontend,kumar303/addons-frontend,tsl143/addons-frontend,mozilla/addons-frontend,mozilla/addons-frontend,mozilla/addons-frontend,aviarypl/mozilla-l10n-addons-frontend,tsl143/addons-frontend,kumar303/addons-frontend,tsl143/addons-frontend,mozilla/addons-frontend,aviarypl/mozilla-l10n-addons-frontend,tsl143/addons-frontend,aviarypl/mozilla-l10n-addons-frontend,aviarypl/mozilla-l10n-addons-frontend