Raven should not choke on circular references

... even if the user provides them.

```
Stacktrace (most recent call first):

  File "fs.js", line 297, in Object.oncomplete
  File "fs.js", line 176, in fs.readFile
  File "/home/ec2-user/node_modules/raven/lib/utils.js", line 148, in Object.<anonymous>
    if (--callbacks === 0) cb(frames);
  File "/home/ec2-user/node_modules/raven/lib/parsers.js", line 27, in module.exports.parseError
    cb(kwargs);
  File "/home/ec2-user/node_modules/raven/lib/client.js", line 123, in _.captureError._.captureException
    var result = self.process(kw);
  File "/home/ec2-user/node_modules/raven/lib/client.js", line 72, in process
    this._enabled && this.send(kwargs);
  File "/home/ec2-user/node_modules/raven/lib/client.js", line 79, in send
    zlib.deflate(JSON.stringify(kwargs), function(err, buff) {
  File "native json.js", line 310, in stringify
```