Kramdown use enable_coderay instead of use_coderay for syntax highlighting

By default, Jekyll set use_coderay to false while Kramdown set enable_coderay to true, so Coderay will be always enabled although use_coderay is set false.

This patch fixed the bug without losing compatibility.

ref: http://kramdown.gettalong.org/options.html
Why not just change the `if` to `enable_coderay`?