Set default message renderer to default renderer

This is the fix to the feature request proposed yesterday (from my company's support window).

I think it is better to default the renderer of Redmine messages to plain text because Redmine messages do not necessarily use Markdown. In versions prior to 2.5 Redmine does not support Markdown and uses Textile instead (this is my company's case). Also, posts often begin with an issue number starting with a `#`, which leads to rendering large characters in the chat window.
It may be better if users can select which behavior to use from Redmine's plugin setting window; I will try it later.