build `rpm` packages

Since it seems PR !44 is dead/abandoned, I'm resurrecting it here with an updated gulpfile.

I'd rather change Linux to Debian (DEB_ARTEFACTS_TEMPLATE v. LINUX_ARTEFACTS_TEMPLATE) and make that explicit but I'm afraid that will break CI (as mentioned in the original PR) and make it harder to have this PR accepted... so I just took the changes from PR #44 as-is.

Tested with Gitter 3.1.0 on Fedora 26 64-bit.