Validate README format before displaying

Take the first previewable README file as project’s README, otherwise
if none file is available, or we can’t preview any of them, we assume
that project doesn’t have a README file.

app/models/tree.rb

@@ -17,18 +17,16 @@ class Tree
   def readme
     return @readme if defined?(@readme)
 
-    available_readmes = blobs.select(&:readme?)
+    # Take the first previewable readme, or return nil if none is available or
+    # we can't preview any of them
+    readme_tree = blobs.find do |blob|
+      blob.readme? && (previewable?(blob.name) || plain?(blob.name))
+    end
 
-    if available_readmes.count == 0
+    if readme_tree.nil?
       return @readme = nil
     end
 
-    # Take the first previewable readme, or the first available readme, if we
-    # can't preview any of them
-    readme_tree = available_readmes.find do |readme|
-      previewable?(readme.name)
-    end || available_readmes.first
-
     readme_path = path == '/' ? readme_tree.name : File.join(path, readme_tree.name)
 
     git_repo = repository.raw_repository