[ci skip] Add reference to `phantomjs` in README
[ci skip] Add reference to `phantomjs` in README

- Actually in order to be able to run integration tests we need to have
`phantomjs` available in the `PATH`. That's required by `capybara` which
is required by `prof`.

[#134349577]

Signed-off-by: Diego Lemos <48d75b1e8d05d722a97d5c3ce49024e6a466bebe@pivotal.io>

pivotal-cf/cf-rabbitmq-release,pivotal-cf/cf-rabbitmq-release,pivotal-cf/cf-rabbitmq-release,pivotal-cf/cf-rabbitmq-release