enable css-nano safe mode

Author: Robert Pritzkow

lib/gulp/tasks/styles.js

@@ -55,6 +55,7 @@ module.exports = function(settings, plugins, config) {
           minifyGradients: true,
           uniqueSelectors: true,
           zindex: false,
+          safe: true,
         },
       ];
       return runner('cssnano', defaults, settings, plugins);