fix(Railtie) make sure initializers always run

[rmosolgo]

It seems like we have a problem with initializers:

• not reloading when testing React components not reloading when testing #155
• "Addons is undefined" TypeError: React.addons is undefined #148
• react source files aren't copied initializer react_rails.setup_vendor fails if gem source files were read-only #175

Messing with the initializers has been recommended before:

• changing the group: Initialize railtie for all groups #40
• initialize after sprockets Fix asset path munging to work in production env. #36 (However that initializer no longer exists)

But I couldn't find much about how initializers run, just an offhand mention in the Railtie docs (do you know of anything better?).

I did find a couple of examples of group: :all in the wild:

• sass-rails does it
• ember-rails does it

I think it's worth a try, you?

[vipulnsward]

LGTM