DRY up the skipping of excluded environments

This is a quick and dirty attempt to reduce the repetition of the same log message and condition checking that I introduced in getsentry/raven-ruby#147. Basically all I’m doing here is:

- Passing a block to a `skip_or_send` method which centralises the `send_in_current_environment?` check.
- Move the debug message logging so that it’s the responsibility of the `Configuration` class, meaning there’s only one place to go if the message needs updating.

/cc @dcramer @nateberkopec Does this help at all? or does it give you any other ideas about how it could be tidied up more? I’m not attached to these method names at all, so let me know if there’s a better way to name/arrange things too :smile: