Automatically generate a debian package

Hi,

Following the previous pull request (#1505) to integrate discourse with the `pkgr` tool (http://github.com/crohr/pkgr), here is the updated configuration so that the latest version of discourse can be successfully packaged for debian (see https://deb.pkgr.io/pkgr/discourse).

For now I have to rebase from discourse/discourse and push to pkgr/discourse to get new packages built, but if you wish you can sign in and get the webhook installed to discourse/discourse so that packages get built automatically. Disclaimer: pkgr.io is still in the testing phase.

I would love to hear what you think!