Support for rails travel methods

There are new helpers since rails 4.1: `travel`, `travel_to` and `travel_back`.
https://github.com/rails/rails/commit/225cd915cf5ab3b6662d3eecf3fe0715bf73e4de

I think they should be available out of the box in rspec, but I'm not sure what would be the best way to include them.

For me, simple line in spec_helper works fine:

```ruby
RSpec.configure do |config|
  config.include ActiveSupport::Testing::TimeHelpers
end
```

It could be included in pregenerated spec_helper (this way user can always disable it) or placed in https://github.com/rspec/rspec-rails/blob/v3.0.0.rc1/lib/rspec/rails/configuration.rb

What do you think?