message: [changelog] 1.6.4
diff: ## Changelog
  +## [v1.6.4](https://github.com/youzan/vant/tree/v1.6.4)
  +`2019-02-14`
  +
  +**Bug Fixes**
  +
  +- Sku: buy event incorrect triggered [\#2741](https://github.com/youzan/vant/pull/2741)
  +
  +
  ## [v1.6.3](https://github.com/youzan/vant/tree/v1.6.3)
  `2019-02-14`
  