Use event_id instead of id if async+thread enabled

[if1live]

How to raise bug

# use you dsn
$ bundle exec rake "raven:test[https://foo:bar@app.getsentry.com/1234]"

Client configuration:
-> server: https://app.getsentry.com
-> project_id: 1234
-> public_key: blabla...
-> secret_key: blabla...

Sending a test event:
rake aborted!
NoMethodError: undefined method `id' for #<Hash:0x007f71af480fa8>

why occur?

When async is enabled, evt.value is like...

{:event_id=>"401ed3befbb0bb5d10807213b8d35c1a", :message=>"ZeroDivisionError: divided by 0", :timestamp=>"2015-05-01T07:55:15", ....

1. evt.value is not object. It is a hash
2. event_id, not id

Related issue / pull request

• https://github.com/getsentry/raven-ruby/issues/203