Removed phantomjs support

[mikhaillopandia]

#274

[coveralls]

Coverage remained the same at 100.0% when pulling dddecea on #274 into 481ea5e on master.

[mikhaillopandia]

Selenium doesn't support PhantomJS anymore:

For Opera, users should be able to simply rely on testing Chrome as the Opera browser is based on Chromium (and the operadriver was a thin wrapper around chromedriver). 
For PhantomJS, users should use Chrome or Firefox in headless mode (see example/headless.js)

https://github.com/SeleniumHQ/selenium/blob/master/javascript/node/selenium-webdriver/CHANGES.md#changes-to-supported-browsers

[romikoops]

I am not sure about Selenium+Firefox as default for the framework. I would use either headless_chrome or poltergeist