drop thin support

[namusyaka]

As per rack/rack@98d9cf5, it's time to drop the support for thin.
Does anyone want to take this issue?

[rkmathi]

@namusyaka Hi
I fixed it to try in #1627.
If I'm wrong, I'd be happy to let you know.

[matti]

I never managed to get streaming work with puma, only with thin. Worried.

[namusyaka]

I guess the rainbows can handle the streaming feature. See #1627

[esparta]

May I propose to re-open this issue in order to link the update of documentation on different languages?
once all the docs are done, close it?

[dentarg]

@esparta I would suggest creating a new issue about the docs (easier to understand what's need to be done then)

[namusyaka]

As I noted in sinatra/sinatra.github.com#274, we'll wait and see what happens around rack/thin for making a decision on what server should be choosed. This indicates there is a possibility of reverting #1627 partially (In any case the integration test with rainbows can be helpful).

[namusyaka]

We'll use rainbows in our development environment, but I restored thin support in 342758a because of macournoyer/thin@b201525.
However, in order to use thin with sinatra smoothly, macournoyer/thin#369 should be merged.