message: [changelog] 1.6.8
diff: ## Changelog
  +## [v1.6.8](https://github.com/youzan/vant/tree/v1.6.8)
  +`2019-03-02`
  +
  +**Improvements**
  +
  +- CouponList: optimize style of empty tip [\#2878](https://github.com/youzan/vant/pull/2878)
  +- allow use ref in functional components [\#2863](https://github.com/youzan/vant/pull/2863)
  +
  +**Bug Fixes**
  +
  +- fix Field label slot not work [\#2872](https://github.com/youzan/vant/pull/2872)
  +- fix Icon missing new icon [\#2887](https://github.com/youzan/vant/pull/2887)
  +- fix AddressList should hide radio when unswitchable [\#2886](https://github.com/youzan/vant/pull/2886)
  +- fix Tab scroll to correct position when use offset-top [\#2873](https://github.com/youzan/vant/pull/2873)
  +
  +
  ## [v1.6.7](https://github.com/youzan/vant/tree/v1.6.7)
  `2019-02-26`
  