Track funnel events with Segment.io

* Base approach on http://robots.thoughtbot.com/segment-io-and-ruby.
* Add `utm_` campaigns as properties on user creation so cohort analysis can be
  done on a per-campaign basis later on.
* Use special Segment.io `revenue` property and special `campaign` context.
  https://segment.io/docs/tracking-api/track/
* Ignore Redis backup dump file.

https://trello.com/c/gwFd8KCN
Configure Segment.io Ruby gem
Improve configuration of Segment.io gem
Upgrade to Rails 4.1.5

* upgrade to RSpec 3
* upgrade to Capybara 2.4.1
* upgrade to capybara-webkit 1.3.0
* use new RSpec 3 syntax for message stubbing