(chore) bump version and publish 0.6.15
(chore) bump version and publish 0.6.15

tempbottle/handlebars-rust,tailhook/handlebars-rust,sunng87/handlebars-rust,wang19742008/handlebars-rust,tempbottle/handlebars-rust,tailhook/handlebars-rust,ihrwein/handlebars-rust,sunng87/handlebars-rust