Problem
You are trying to install spree from the git source, following the instructions from here, but there are errors like:

gem install activemerchant --version "= 1.4.1"
ERROR:  While generating documentation for builder-2.1.2
... MESSAGE:   Unhandled special: Special: type=17, text=""
... RDOC args: --ri --op /usr/lib/ruby/gems/1.8/doc/builder-2.1.2/ri --title Builder
    -- Easy XML Building --main README --line-numbers
    --quiet lib CHANGES Rakefile README doc/releases/builder-1.2.4.rdoc
    doc/releases/builder-2.0.0.rdoc doc/releases/builder-2.1.1.

Solution
Make sure you have the following gems installed:

sudo gem install builder haml echoe

and then run:

sudo rake gems:install