Removes duplicate mkdir_p

Rakefile

@@ -101,7 +101,6 @@ task :new_post, :title do |t, args|
   end
   puts "Creating new post: #{filename}"
   open(filename, 'w') do |post|
-    system "mkdir -p #{source_dir}/#{posts_dir}/";
     post.puts "---"
     post.puts "layout: post"
     post.puts "title: \"#{title.gsub(/&/,'&').titlecase}\""