I have geckodriver installed in my development directory
ls | grep geckodriver
geckodriver
geckodriver-v0.11.1-linux64.tar.gz
I also exported
You can now move toselenium-webdriver 3.4.3 which is the latest version. However, you still need the GeckoDriver because there no native extensions of Selenium in FF. You can also add the geckodriver-helper gem to your GemFile. Check https://github.com/DevicoSolutions/geckodriver-helper .
Be aware of that there are still some issues that have to be fixed when you use the combination of Selenium3 & FF48+ combination and can affect your tests like double-click and modal windows.