Prevent a NameError when loading outside of Rails

When running `bundle console` we get this error:

```
>>: bundle console
Resolving dependencies...
./lib/react/rails/view_helper.rb:27:in `<top (required)>': uninitialized constant ActionView (NameError)
...
```

I could have also checked with `defined? ActionView::Base` before including, but this would mean that if `ActionView::Base` was defined elsewhere later, the react rails helpers would not be included.