Cast remark's vfile to string

Another change on the API was the change of the return type.  It now return a vfile instead of a string.

Old:
d1c52a3b28/doc/remark.3.md (remarkprocessfile-options-done)

New:
https://github.com/wooorm/unified#processorprocessfilevalue-options-done

src/beautifiers/remark.coffee

@@ -36,7 +36,7 @@ module.exports = class Remark extends Beautifier
     return new @Promise((resolve, reject) ->
       try
         remark = require 'remark'
-        cleanMarkdown = remark().process(text, options)
+        cleanMarkdown = remark().process(text, options).toString()
         resolve cleanMarkdown
       catch err
         @error("Remark error: #{err}")