Add 'phantomjs' driver support for selenium ~> 3.8.0

[zozivan]

Meta

Howitzer Version: 2.1.0

Expected Behavior

There should be phantomjs directory in selenium/webdriver directory

Actual Behavior

There is no phantomjs support for selenium ~> 3.8.0

Steps to reproduce

1. Install new project of howitzer 2.1.0
2. Create howitzer gemset.
3. Open External libraries -> selenium-webdriver -> lib -> selenium -> webdriver

[mikhaillopandia]

Selenium has officially stopped support of phantomjs:

PhantomJS:
  * Support is deprecated in favor of headless Chrome/Firefox or HTMLUnit.
    PhantomJS is no longer actively developed, and support will eventually
    be dropped.

https://github.com/SeleniumHQ/selenium/blob/7f66c20dde7a4e939927b678f585d0586f160ca6/rb/CHANGES#L79

So we are going to stop support of phantomjs in Howitzer as well in new release.

But if you want to run your tests in phantomjs with Howitzer 2.1.0, you may use selenium-webdriver version with phantomjs support, i.e. 3.7.0 and just need to add string

gem 'selenium-webdriver', '3.7.0'

in your Gemfile and run

bundle update selenium-webdriver