Update staff directory for Ruby 2.1.1

This updates the House Staff Directory to use Ruby 2.1.1, and more modern deployment practices.

* Removes the deprecated `sunlight` gem and replaces it with the [congress](https://rubygems.org/gems/congress) gem.
* This has the effect of switching the loading tasks to use the new [Sunlight Congress API](https://sunlightlabs.github.io/congress/) instead of the old one. This also fixes some UTF-8 errors.
* Ruby 1.9+: updated requires of local files to use local paths, e.g. `require './staffers'`.
* Ruby 1.9+: replaced `fastercsv` with `csv`.
* Uses a unicorn.rb file so that multiple unicorn workers get started in production.
* Uses [fabric](http://fabric.readthedocs.org/en/1.8/) for deployment.
* [rbenv](https://github.com/sstephenson/rbenv) is now being used on the production server, and obeying `.ruby-version`.
* Uses [safe_yaml](https://github.com/dtao/safe_yaml#safeyaml) to prevent any YAML vulnerabilities.
* Also, a bug fix for a search regression in https://github.com/sunlightlabs/staffers/commit/99501313a33e65a35aecb2cfba60d631cae65d1d that meant last-name-only searches didn't work.

This didn't do any major version upgrades for individual gems. For example, this still uses Mongoid 2, and Mongoid's soon to release version 4. The purpose of this PR is to ease *managing our deployment*, not to develop the app itself further.