Remove phantomjs to use chrome headless

[cedvan]

• PhantomJS is deprecated : cf. https://groups.google.com/forum/m/#!topic/phantomjs/9aI5d-LDuNE
• Chrome headless come with real chrome engine

[mageekguy]

Yep, but it seems that chrome headless can be very slow in some configuration.
However i got this feeling from several months ago, so in my opinion, we must check the state of performance of chrome headless before go ahead.

[borisfournier]

• I found this article : https://hackernoon.com/benchmark-headless-chrome-vs-phantomjs-e7f44c6956c
• Coming with this tool : https://github.com/hartator/benchmark-headless-chrome-vs-phantomjs
  So I tried on my own computer to have more results to be able to compare and here are my results :

Computer: Dell Latitude E5500 (2.3 GHz and 7.7 GB of Ram) using Fedora28, with Ruby ruby 2.5.3p105, Chrome 70.0.3538.102 and PhantomJS 2.1.1.

The Less the Better. I've run the test 10 times for each benchmark and checked the execution time in seconds and the CPU consumption in percentage:

My results are quite different from the article I found and it appears that on my computer Chrome-headless is slower and greedier.