Refactor configuration implementation

The previous configuration implementation was needlessly complicated.
This commit simplifies the configuration by:

1. Setting configuration defaults in RemoteDatabasCleaner::Config#initialize,
   and removing the need for the gem 'Virtus'
2. RemoteDatabaseCleaner.configure 'yields' the configuration object rather
   than constructing a hash via an OpenStruct object.
3. Removes RemoteDatabaseCleaner::Config.configure. Removing this method
   encourages users to configure remote_database_cleaner though
   RemoteDatabaseCleaner object.