Skip excluded environments before initializing events

This is a quick and dirty attempt to fix the issue I recently described in getsentry/raven-ruby#146. I’m basically just duplicating [these few lines here](https://github.com/getsentry/raven-ruby/blob/master/lib/raven/client.rb#L25-L28), but doing the check before the actual `Event` gets created, which is what I believe to be causing the long delay in development. I’m sure there’s better ways to do this, so I’m open to alternative ideas :grin: 

@dcramer Not sure what the best way is to reduce the duplication here. Any suggestions?