fixup! seleniumrunner: repair compatibility with erp5testnode

geckodriver is also required

software/seleniumrunner/software.cfg

@@ -19,7 +19,9 @@ parts =
    slapos-cookbook
    template
    firefox-wrapper
-# seleniumserver needs to install a default firefox at ${buildout:bin-directory}/firefox for erp5testnode
+   geckodriver
+# seleniumserver needs to install a default firefox as ${buildout:bin-directory}/firefox and
+# a geckodriver as ${buildout:bin-directory}/geckodriver for erp5testnode
 
 [instance-recipe]
 egg = slapos.cookbook