Dynamically require combustion/application only in #initialize!

[jrochkind]

To avoid Rails 8.2 alpha load-order warnings when config/application is loaded earlier and registers initializers before Rails framework has been loaded properly.

Closes #144

[jrochkind]

I'm not sure if the build errors are related to my change or not; perhaps I have indeed messed up the setup for test?

An error occurred while loading spec_helper.rb.
Failure/Error:
  ActiveRecord::Base.configurations = YAML.safe_load(
    ERB.new(database_yaml).result, :aliases => true
  )

ActiveRecord::AdapterNotSpecified:
  'development' database is not configured. Available: ["test", "test_another"]

To the extent I can get tests to run locally -- it was a bit tricky to figure out how -- I don't seem to be able to reproduce the failure. I have run local tests with sqlite3 and had them pass.

Advice welcome!