Added explicit order to variables returned by the database in variable_features_export_render

When exporting variables using Strongarm via Features, although Features reports that no variables have changed, when downloading the feature and performing a diff, the Features Strongarm .inc file can potentially report many changes, especially when multiple developers are contributing to the feature from many different databases. This is because the database call in the variable_features_export_render() function relies on the databases sorting of the records returned. Always ordering variables by name would make diffing much easier!