chore(package): update fetch-mock to version 5.9.3
chore(package): update fetch-mock to version 5.9.3

Closes #1671

https://greenkeeper.io/

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