fix ace editor Syntax Highlighter mode load 404

This error happens on `production mode` only, when you edit a markdown file, the content can't be correctly highlighted cause of a `404` error of ajax loading `mode-markdown.js`.

Ace load js use a relative path to itself and our `ace.js` is packed to `application.js`, so the final url ace used to load js is error.

I use nginx `gzip_static` module, so it's better to precompile all ace related js: 
```ruby
config.assets.precompile += %w( ace-src-noconflict/* )
```