Change custom precompile to check ReactOnRails.configuration.build_production_command present?

[justin808]

Update logic on what build command to use in assets:precompile to be based on whether or not
config.build_production_command is defined.

This change is 

[justin808]

@gscarv13 pick this up.

[justin808]

Reviewed 1 of 2 files at r1, 4 of 4 files at r2, all commit messages.
Reviewable status: all files reviewed, 4 unresolved discussions

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CHANGELOG.md, line 21 at r2 (raw file):

#### Changed
- Remove the documentation entries that asked to delete the `config/webpack/production.js` file if `config.build_production_command` is present [PR 1402](https://github.com/shakacode/react_on_rails/pull/1402)by [justin808](https://github.com/justin808) and [gscarv13](https://github.com/gscarv13).

Changed logic of determining the usage of the default rails/webpacker webpack config or a custom command to only check if the config.build_production_command is defined.

The point of the PR is to change the logic. We don't update the changelog with documentation commands.

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docs/deployment/heroku-deployment.md, line 24 at r2 (raw file):

If you're a custom webpack configuration, and you **do not have the default
`config/webpack/production.js`** file, then the `config/initializers/react_on_rails.rb`
configuration `config.build_production_command` will be used.

Reword this for the change in logic.

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docs/guides/configuration.md, line 70 at r2 (raw file):

if you don't want react_on_rails building this file for you.

if you don't want react_on_rails invoking this command

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lib/tasks/assets.rake, line 12 at r2 (raw file):

skip_react_on_rails_precompile = %w[no false n f].include?(ENV["REACT_ON_RAILS_PRECOMPILE"])

if !skip_react_on_rails_precompile && ReactOnRails.configuration.build_production_command.present?

Test this logic for 4 cases:

1. build_production_command script set, no /config/webpack/production.js exists ==> runs script
2. build_production_command script set, and /config/webpack/production.js exists ==> runs script
3. build_production_command script not set, and /config/webpack/production.js exists ==> runs this
4. build_production_command script not set, and /config/webpack/production.js not existing ==> prints warning message? (check what it does)