Fix mtime usage in compileCSS

mainly fix the problem in the few lines below, the mtime is undefined in the first `if` statement.
```coffee-script
        stats = fs.statSync @absPath(sourcePath)
        if ext is 'css'
          if timeEq mtime, @cache.map[route]?.mtime
            alreadyCached = true
          else
            {mtime} = stats
            css = (fs.readFileSync @absPath(sourcePath)).toString 'utf8'
            css = @fixCSSImagePaths css
```