The orderby value you’re using doesn’t really work.
You should use either 'orderby' => 'meta_value' or 'orderby' => 'meta_value_num'
WP 4.2 also introduced a better way to order by meta_query fields that is detailed here https://make.wordpress.org/core/2015/03/30/query-improvements-in-wp-4-2-orderby-and-meta_query/
