Stop guessing the GeckoProfiler API

[tarekziade]

Why are we doing guesswork here by counting the number of arguments the API has ?

https://github.com/mozilla/browsertime/blob/master/lib/firefox/webdriver/firefoxDelegate.js#L54-L74
this leads to a fragile implementation, that gets broken everytime the API is changed, see #50

we should explicitely look at the firefox version and implement the right script for each supported Firefox version (and also throw a hard error in case it's not implemented. no need to go further like we do now)