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

Created by: dfyx

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.