Javascript testing a Symfony 3 application with Mink and Selenium - php

Question

I am trying to test some javascript functionality in my Symfony Application. I tried looking at Mink for this:
http://mink.behat.org/en/latest/.
I ran a composer require for behat/mink, behat/mink-goutte-driver, and behat/mink-selenium2-driver.
I then tried to install Selenium in my ubuntu 16.04 dev enviroment.
I've got a bash script that installs java8, selenium 3.9, openjdk-8-jdk-headless, firefox, xvfb and geckodriver 0.18.0. I seem to start up the selenium server OK.
However, when I then try to start a session with Mink in my tests, I am seeing the following:
Behat\Mink\Exception\DriverException: Could not open connection: Unable to find a matching set of capabilities
Build info: version: '3.9.0', revision: '698b3178f0', time: '2018-02-05T14:56:13.134Z'
System info: os.name: 'Linux', os.arch: 'amd64', os.version: '4.9.60-linuxkit-aufs', java.version: '1.8.0_161'
Driver info: driver.version: unknown
Caused by
WebDriver\Exception\SessionNotCreated: Unable to find a matching set of capabilities
Build info: version: '3.9.0', revision: '698b3178f0', time: '2018-02-05T14:56:13.134Z'
System info: os.name: 'Linux', os.arch: 'amd64', os.version: '4.9.60-linuxkit-aufs', java.version: '1.8.0_161'
Driver info: driver.version: unknown
I'm trying to start my session in my test class like this:
// Choose a Mink driver. More about it in later chapters.
$driver = new \Behat\Mink\Driver\Selenium2Driver();
$session = new \Behat\Mink\Session($driver);
// start the session
$session->start();
I assume my problem is to do with the versions of java/firefox/selenium/ geckodriver incompatible with each other? But I can't seem to be able to fix it by trying out other versions... Is anyone able to help?
(This is my first ever question here, so let me know if if isn't great and I will try to improve it!)