Santizing of URL not needed 

Hi Ben,

I noticed that the URLs in the Airbrake interface were being html sanitized. Hence they were including lots of `&amp` etc. As far as I can tell we don't need to sanitize the URLs so I have done another modification to [`core.clj`](https://github.com/jasonneylon/clj-airbrake/blob/528b2f86caafd9170ce02cb34660884c82b11cce/src/clj_airbrake/core.clj#L71) to reflect this. I have tested this with `<`, `>`, `&` characters in the url and they are successfully processed by Airbrake.

Thanks,
Jason