tests: use latest Safari in Saucelab front end tests
tests: use latest Safari in Saucelab front end tests

I verified that, as of today, "latest" Safari version works on Saucelabs.

storytouch/etherpad-lite,storytouch/etherpad-lite,aptivate/etherpad-lite,ether/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite,storytouch/etherpad-lite,aptivate/etherpad-lite,ether/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite,joequant/etherpad-lite