Fix `jekyll serve --limit_posts n` failed

Just fix this error:

```
$ ./bin/jekyll serve --limit_posts 2 --trace
Configuration file: /home/nyaran/blog.uu59.org/_config.yml
/home/nyaran/.rbenv/versions/1.9.3-p374/lib/ruby/gems/1.9.1/bundler/gems/jekyll-c0c0150bb5a3/lib/jekyll/site.rb:65:in `<': comparison of String with 1 failed (ArgumentError)

```