Adds puma and moves thin in Gemfile

* Added puma to Gemfile

Puma is the next server for production.

* Moved thin to :development group

thin will not needed in production, since it's superseeded by Puma.
It is hard to deal with long running code under thin. Since this long
running code is hard to make evented and sinc we use only one process
per application/thin, Puma is a better choice for now.