Fix config.server_bundle_js file in generator

[justin808]

We're setting the config.server_bundle_js file even if server rendering is not enabled.

https://github.com/shakacode/react_on_rails/blob/master/lib%2Fgenerators%2Freact_on_rails%2Ftemplates%2Fbase%2Fbase%2Fconfig%2Finitializers%2Freact_on_rails.rb#L7

# Shown below are the defaults for configuration
ReactOnRails.configure do |config|
  # Client bundles are configured in application.js

  # Server rendering:
  # Server bundle is a single file for all server rendering of components.
  config.server_bundle_js_file = "app/assets/javascripts/generated/server-bundle.js"

We need to templatize line 7 in that file.

Here's a crystal clear example:

https://github.com/shakacode/react_on_rails/blob/master/lib%2Fgenerators%2Freact_on_rails%2Ftemplates%2Fbase%2Fbase%2FProcfile.dev.tt

<%- if options.server_rendering? %>server: sh -c 'cd client && npm run build:dev:server'<%- end %>