Before this PR, `use_parent_strategy` was set on the configuration
instance. Since `FactoryBot.reload` wipes out the configuration, it also
ends up resetting `use_parent_strategy` back to `nil`.
This can cause problems when using factory_bot_rails with Spring, since
it calls `FactoryBot.reload` each time Spring forks. If
`use_parent_strategy` is set in a file that Spring preloads, like in an
initializer, then the value will get wiped out.
With this PR, we set `use_parent_strategy` directly on FactoryBot,
rather than on the configuration instance, and so reloading no longer
has any effect.
3e1a941347