What does this error while running 'middleman' mean?

I came across this middleman package
GitHub - thenickcox/Middleman-Skeleton: A port of the Skeleton Responsive framework for use with Middleman, HAML, and Sass and tried to run it
with bundle exec middleman server but get this back in the terminal…

The source :rubygems is deprecated because HTTP requests are

insecure. Please change your source to ‘https://rubygems.org’ if
possible, or ‘http://rubygems.org’ if not.

== The Middleman is loading
== Port 4567 is unavailable. Either close the instance of Middleman

already running on 4567 or start this Middleman on a new port with:

Any help would be helpful!