Only include lib files in the gem

Resolves https://github.com/jekyll/jekyll/issues/2670

jekyll.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |s|
   s.email    = 'tom@mojombo.com'
   s.homepage = 'https://github.com/jekyll/jekyll'
 
-  s.files         = `git ls-files`.split($/)
+  s.files         = Dir["lib/**/*.rb"]
   s.executables   = s.files.grep(%r{^bin/}) { |f| File.basename(f) }
   s.test_files    = s.files.grep(%r{^(test|spec|features)/})
   s.require_paths = ["lib"]