Use Thin in development and Unicorn in staging and production.

This PR is sent because, currently we use Unicorn server for development env as well. But settings for Unicorn is not good for development env.

https://github.com/swjg-ventures/myautobrain/blob/df2ba03f250c6c2a031b1b696255095a8eacc814/config/unicorn.rb

- We can't use debugger properly because timeout is set for 30 seconds.
- No need of worker_processes set to 2 for development.

Either we can modify these settings based on dev and staging/production or we can use Thin directly.