Add Gemfast

[gscho]

A drop in replacement for gem in a box written in Go: https://github.com/gemfast/server

There probably should be a section dedicated to gem servers including:

• gemstash (https://github.com/rubygems/gemstash)
• gem in a box (https://github.com/geminabox/geminabox)
• gemirro (https://github.com/PierreRambaud/gemirro)

[gscho]

Thanks!