I feel like a dolt for not knowing this, but what is the command to install the latest version?
'gem install middleman' delivers 3.0.13 (as it presumably should).
I am running ruby 2.0.0p0
gem install middleman --pre
However, you should be able to do this in a Gemfile for an existing project rather than using a system-wide install.
Thank you. I updated my project gemfile to say:
gem "middleman", "~>3.1.0.beta"
This appears to be the preferred way to implement "--pre" in a gemfile.