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 4.1.0

This brought in a Rspec 3 upgrade and capybara-webkit 1.3.0 upgrade.

One of the tests is failing: spec/features/repo_list_spec.rb, "user
syncs repos". It pops up a JS alert "Your repos failed to sync" because
an AJAX request fails; this alert is unexpected.
Do not use the explicit promose for Sync.save

This come from the Rails upgrade, which pushed forward an AngularJS
upgrade.
Use RSpec 3 stubbing

The old #stub method is deprecated.
wip
wip
wip
use the default port