Migrate to RSpec 3 and eliminate all deprecations

* Add RSpec 3.x rails_helper and spec_helper conventions
* Use the `expect` syntax over the old `should` syntax
* Use the `allow` syntax over the old `mock` syntax
* Remove `visit` in controller specs
* Change :example_group filtering
* Change `expect(record).not_to have(1).error_on(:slug)` syntax
* w/ capybara upgraded from `0.2` to `2.3`
* w/ cancan upgraded to `cancancan`