Fix rendering of README.txt on tree page. See #1676

*I posted an incorrect Pull Request a few minutes ago. Ignore that one. This is the correct one when merging into master*

Previously, only files that where named exactly "README" were rendered as plaintext. Files named "Readme", "README.txt" or something similar were incorrectly handled as Markdown which resulted in a missing `<pre>` tag

Consider this to be a quick and dirty fix. A cleaner version would be to share a partial between `app/views/tree/_tree.html.haml` and `app/views/blob/_text.html.haml` instead of having `app/views/tree/_readme.html.haml`. This woukd ensure that file rendering is consistent between those two pages but I'm not that fluent in HAML and your testing workflow.