view: properly hide labels of the datetime selector

The accessibility labels where not properly hidden, which
resulted in the labels being visible and stacked vertically.

config/initializers/date_select.rb

@@ -25,7 +25,7 @@ module ActionView
         end
         field_for = "#{prefix.join('_')}_#{@options[:field_name]}"
 
-        "<label class='screen-reader-text' for='#{field_for}_#{n}i'>#{label}</label>"
+        "<label class='sr-only' for='#{field_for}_#{n}i'>#{label}</label>"
       end
 
       # Returns the separator for a given datetime component.