Fix Rakefile to prevent 'uninitialized constant CLEAN'

Author: David Smyth

Rakefile

@@ -1,4 +1,5 @@
 require 'rake'
+require 'rake/clean'
 load 'thin.gemspec'
 
 # Load tasks in tasks/
@@ -21,4 +22,4 @@ task :install => :build do
 end
 
 desc "Release version #{Thin::VERSION::STRING}"
-task :release => [:tag, :push]
+task :release => [:tag, :push]