Try to support Redmine 5

Author: sanak

app/views/gtt_map_layers/index.html.erb

@@ -1,5 +1,5 @@
 <div class="contextual">
-<%= link_to sprite_icon('add', t('map_layer.new')), new_gtt_map_layer_path, :class => 'icon icon-add' %>
+<%= link_to (Redmine::VERSION.to_s >= '6.0.0') ? sprite_icon('add', t('map_layer.new')) : t('map_layer.new'), new_gtt_map_layer_path, :class => 'icon icon-add' %>
 </div>
 
 <%= title t('map_layer.plural') %>