Replace PhantomJS with headless Chrome

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FE @mikegreiling

Chrome 59 can now be run with --headless on all platforms. https://www.chromestatus.com/features/5678767817097216

We should look into replacing PhantomJS with headless Chrome in our GitLab tests for more accurate and less error-prone integration testing.

This needs to be done in two places:

• Update docker images to include headless Chrome pre-installed (gitlab-build-images!41 (merged))
• Remove PhantomJS from these images
• Update Karma config to utilize Chrome instead of PhantomJS (gitlab-ce!12036)
  • should require a simple change to karma-chrome-launcher
  • Remove PhantomJS-specific hacks from our frontend tests
• Update RSpec config to replace Poltergeist with a headless Chrome solution that works with Capybara
  • could possibly use the selenium driver for this?