Try updating PHPUnit dependencies. May require PHPUnit 8.

Author: colinmollenhour

dev/tests/functional/composer.json

@@ -11,10 +11,11 @@
     ],
     "require": {
         "magento/mtf": "1.0.0-rc64",
-        "php": ">=5.4.0",
-        "phpunit/phpunit": "4.1.0",
-        "phpunit/phpunit-selenium": ">=1.2",
-        "netwing/selenium-server-standalone": ">=2.35"
+        "php": "~7.1.0||~7.2.0||~7.3.0",
+        "phpunit/phpunit": "~6.5.0",
+        "phpunit/phpunit-selenium": "~4.1.0",
+        "php-webdriver/webdriver":  "~1.8.2",
+        "se/selenium-server-standalone": "2.53.1"
     },
     "autoload": {
         "psr-4": {

dev/travis/before_script.sh

@@ -24,7 +24,7 @@ case $TEST_SUITE in
         echo "Prepare functional tests for running"
         cd dev/tests/functional
 
-        composer install && composer require se/selenium-server-standalone:2.53.1
+        composer install  --no-interaction
         export DISPLAY=:1.0
         sh ./vendor/se/selenium-server-standalone/bin/selenium-server-standalone -port 4444 -host 127.0.0.1 \
             -Dwebdriver.firefox.bin=$(which firefox) -trustAllSSLCertificate &> ~/selenium.log &