Set :puma_preload_app to false

[rafaelgoulart]

Since the default number of workers is zero, preload_app default config should be false

After reading the puma's README, I noticed they don't recommend to use preload_app if you're not using workers. I think it'd better to set the default to false.

General rule is to use preload_app when your workers die often and need fast starts. If you don’t have many workers, you probably should not use preload_app