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.
@neerajdotname I was going through what @vipulnsward said me today "We should be keeping all environments in sync" and then I found this one http://12factor.net/dev-prod-parity.

Based on this, I think we can continue with using Unicorn server in development too, but for some conditions we can move unicorn settings in settings.yml file.