Fix Rakefile to prevent 'uninitialized constant CLEAN'

When building on ruby-2.0.0-p353 this happens:

% git clone git://github.com/macournoyer/thin.git
% cd thin
% rake install
rake aborted!
uninitialized constant CLEAN
tasks/spec.rake:1:in `<top (required)>'
/home/david.smyth/ruby/fail/thin/Rakefile:5:in `load'
/home/david.smyth/ruby/fail/thin/Rakefile:5:in `block in <top (required)>'
/home/david.smyth/ruby/fail/thin/Rakefile:5:in `each'
/home/david.smyth/ruby/fail/thin/Rakefile:5:in `<top (required)>'
(See full trace by running task with --trace)

Adding the one line to Rakefile fixed the problem.