Support `Pathname` for `deprecation_stream`

Noticed this the other day when I was updating a Rails app; though this has nothing to do with Rails.

```ruby
RSpec.configure do |config|
  config.deprecation_stream = Rails.root.join('tmp', 'deprecations.txt')
end
```

Fails with:

```
rspec-core-3.0.0.beta2/lib/rspec/core/formatters/deprecation_formatter.rb:163:in `print_deferred_deprecation_warnings': private method `puts' called for #<Pathname:0x007fbdc61c9540> (NoMethodError)
```

The workaround is to cast it to a String. I think it's fairly common to provide file paths via `Pathname`. We should support this convention.