Log network traffic on Capybara::Poltergeist::TimeoutError

We've been having a problem with one poltergeist feature which regularly times out.
This monkey-patches poltergeist to catch timeout errors and log the network traffic
before raising, giving us a chance of debugging exactly what's happening.
Merge pull request #1100 from alphagov/log-network-traffic-on-poltergeist-timeout-error

Log network traffic on Capybara::Poltergeist::TimeoutError

[Working towards https://www.pivotaltracker.com/story/show/61376422]