Incompatible with v3 of `connection_pool`

[G-Rath]

v3 changed ConnectionPool to take keyword args rather than positional args, which breaks

https://github.com/reactjs/react-rails/blob/28ee68955f00261aecc07061ea6ea1dfc0831e06/lib/react/server_rendering.rb#L19

I've not dug deeper into this yet so that might not be the only change that is needed - I'd consider doing a new version locking connection_pool to <= 2 for the time being