Support the rails 4.2 (4.2.0beta4 at time of commit)

Rails >= 4.2 has changed the method arity of
_compute_redirect_to_location from (options) to
(*args). The validation inside then expects two
parameters: request, options. This commit handles
these changes.

[Rails 4.2 _compute_redirect_to_location source](https://github.com/rails/rails/blob/2090615d39c071c9eb25e715275eb79f3f9b6266/actionpack/lib/action_controller/metal/redirecting.rb#L79)

Instead of checking the Rails.version, we could check the method arity:

```ruby
if method(: _compute_redirect_to_location).arity == -1
  self.location = _compute_redirect_to_location(request, options)
else ...
```

I was torn, but went with the version check because it was simpler.