Unable to start middleman server

[alvar-o]

Just installed middleman and tumblargh. Followed the instructions on the readme but I'm getting this:

$ bundle exec middleman server
bundler: failed to load command: middleman (/Users/alvaroantonio/.gem/bin/middleman)
Bundler::GemRequireError: There was an error while trying to load the gem 'tumblargh'.
Gem Load Error is: You must provide a Middleman::Extension or a block that returns a Middleman::Extension

Idk if this is still on development, but if you have any idea what might be wrong please answer. Thanks!