env.rb

require_relative 'warning_suppressor'

Capybara.register_driver :poltergeist do |app|
  Capybara::Poltergeist::Driver.new app, phantomjs_logger: WarningSuppressor
end

warning_suppressor.rb

class WarningSuppressor
  class << self
    def write(message)
      if message =~ /QFont::setPixelSize: Pixel size <= 0/ ||
          message =~/CoreText performance note:/
         0
      else
        puts(message)
        1
      end
    end
  end
end