Because of the Poodle vulnerability, some services are disabling SSLv3.
PhanthomJS runs SSLv3 by default (as of version 1.9.7), so you may get unexpected sudden tests fails.
To fix just specify the SSL protocol you want to use instead:
Capybara.register_driver :poltergeist do |app|
phantomjs_options: ['--ssl-protocol= TLSv1']
UPDATE: prefer TLSv1 over ssl2