What does this error while running 'middleman' mean?

I came across this middleman package
https://github.com/thenickcox/Middleman-Skeleton 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!