The SyntaxError seems to be complaining about this inside Middleman:
ProgressBar.create(total: paths.length)
The /var/lib/gems/1.8/
path indicates that you're using Ruby 1.8.X. But the JavaScript-style Hash notation:
total: paths.length
doesn't work in 1.8, you have to use the :total => paths.length
notation there. Looks like you can't use that version of Middleman with 1.8.7, either upgrade to 1.9.3+ or downgrade your Middleman to something that works with older Rubies.