dev build of sentry

The latest stable verison 6.4.4 is missing some [features](https://github.com/getsentry/raven-js/issues/299). We need to be able to use the latest dev version of sentry. 

I've created a separate dev branch. Also, I had to use easy_install as dev egg was not able to be located using pip.

Could you please create a new docker build for the dev sentry and tag the docker image as dev?

Thanks