Drop PADRINO_ENV in favour of RACK_ENV

Hey @padrino/core-members,

I've been thinking about this one for a while and 0.12 might be a good place to introduce the change. I can't really find a reason why we would like to have our own `PADRINO_ENV` variable when we could just reuse `RACK_ENV` and be more compatible with with Sinatra and Rack based apps.

In this PR I'm replacing all the occurrences of `PADRINO_ENV` for `RACK_ENV`.

Thoughts?