bump webdriver to 3.6.0

[Travis Glenn Hansen]

Hey, this project is a great fit for our team. However using recent versions of selenium with firefox seem to fail. The driver is looking for sessionId in the top-level and for some reason it's coming from from the grid with value.sessionId. Chrome seems to work fine along with firefox standalone.

Anyway, I simply bumped the webdriver dep to the latest 3.x series and it seemed to resolve the issue. I tried several different variations going through standalone and grid and everything seemed to work nicely.

I'm also interested in expanding what/how options can get passed down to allow for greater detail in browser selection.

Thanks!