Fix deps for the Ruby + PhantomJS build.
Apparently we need to explicitly redirect output to /dev/null for PhantomJS to work on JRuby.
Add ability to pass :desired_capabilities to all the Ruby drivers.

This fixes issue 5078. Also addresses:

* https://github.com/SeleniumHQ/selenium/pull/8
* https://github.com/SeleniumHQ/selenium/pull/11