Maybe replace Firefly and unpeg Owin and Gate

[bentayloruk]

Cantos is using Firefly as the preview web server. However, Firefly appears to be abandonware.

I've had to do the following to keep Firefly working:

• Peg Gate assembly to 0.3.6 see allowedVersions in packages
• Peg Own assembly to 0.7 see allowedVersions in packages

I did spend a day trying to get Nancy and ServiceStack to do what I want, but they don't do it OOTB, so sticking with Firefly for now (it works enough).