[Dangerfile] Don't try and read directories

Dangerfile

 # Don't let testing shortcuts get into master by accident
 
 (modified_files + added_files - %w(Dangerfile)).each do |file|
+  next unless File.file?(file)
   contents = File.read(file)
   if file.start_with?('spec')
     fail("`xit` or `fit` left in tests (#{file})") if contents =~ /^\w*[xf]it/