react.js not available when config.react.variant is not set

[jtmalinowski]

@zpao I thought we were going to go with environment based variant, if config.react.variant is not explicitly set. However

if variant = app.config.react.variant || ::Rails.env.test?

makes it very clear that it needs to be configured. I think if it is as intended, then we should emphasize it a bit more in the readme. Because that bit even me a few times...

[zpao]

I forgot about that plan, and I've honestly been way too busy with everything else React that I haven't given react-rails any love. I think it would be a great idea to make this more flexible. If you find the time to do it, I wouldn't complain :)

If you have any other features you think would be handy (component generators?), feel free. I would love to see this become more useful for people who are actually using it!