Upgrade to RSpec 3

- Converted RSpec 2-style specs to RSpec 3-style specs using Transpec.
- Disabled the should syntax in favor of the expect syntax for reasons explained [here](http://myronmars.to/n/dev-blog/2012/06/rspecs-new-expectation-syntax).
- Removed the following line from the spec_helper because it is deprecated and is now the default behavior:

```ruby
c.treat_symbols_as_metadata_keys_with_true_values = true
```