Stop packaging test files with the gem

It drastically reduces the size of the gem, from 1.9M to 32K. The
default for new gems (created by bundler) is not to include test files.
See rubygems/bundler#3207

riddle.gemspec

@@ -14,9 +14,7 @@ Gem::Specification.new do |s|
 
   s.rubyforge_project = 'riddle'
 
-  s.files         = `git ls-files`.split("\n")
-  s.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")
-  s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
+  s.files         = `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
   s.require_paths = ['lib']
 
   s.add_development_dependency 'rake',   '>= 0.9.2'