development as default environment

Many projects - Rails, Rack when doesn't detect environment with ENV variables  ( RAILS_ENV, RACK_ENV ) then uses default "development" environment. I think that **raven** should also do it. 

It will fix issue #143 when raven is sending exceptions from local machines. 

I have also found that at beginning raven-ruby had there `'development'` environment set as default. See: e0c84a33