Capybara 3.0 support

[odlp]

Capybara 3.0 has been released so we should try and support it!

Currently the gemspec allows >= 2.7.0, < 3.0.0:

capybara_discoball/capybara_discoball.gemspec

Line 26 in 5cd45cd

s.add_dependency 'capybara', '~> 2.7'

Notably the default server in Capybara is now Puma, but Webrick should still be available:
https://github.com/teamcapybara/capybara/blob/master/UPGRADING.md#server

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Steps:

• Get the build passing
  • It's failing locally for me, I think because aruba 1.0 has been released and there's no Gemfile.lock in this project
• Use appraisal to test Capybara Discoball with Capybara 2.x & 3.0 - Add Capybara 3.0 support #19
• Publish a new version on RubyGems

[odlp]

Released 0.0.3 which supports Capybara 3.0:
https://rubygems.org/gems/capybara_discoball