add date for 3.1.0 change log

CHANGELOG.en-US.md

@@ -17,6 +17,8 @@ timeline: true
 
 ## 3.1.0
 
+`2017-12-29`
+
 - 🐞 Fix ant-spin-container overflow resizing for Spin component. [#8602](https://github.com/ant-design/ant-design/issues/8602) [@jhsu](https://github.com/jhsu)
 - 🐞 Fix extra padding in Table small with fixed columns. [#8724](https://github.com/ant-design/ant-design/issues/8724)
 - 🐞 Fix Checkbox.Group align issue in Form. [#8739](https://github.com/ant-design/ant-design/issues/8739)

CHANGELOG.zh-CN.md

@@ -17,6 +17,8 @@ timeline: true
 
 ## 3.1.0
 
+`2017-12-29`
+
 - 🐞 修复组件 Spin 可能出现跳动的问题。[#8602](https://github.com/ant-design/ant-design/issues/8602) [@jhsu](https://github.com/jhsu)
 - 🐞 修复 Table 在设置 `size=small` 的情况下出现多余 padding 的问题。[#8724](https://github.com/ant-design/ant-design/issues/8724)
 - 🐞 修复 Checkbox.Group 在 Form 中和 label 的对齐问题。[#8739](https://github.com/ant-design/ant-design/issues/8739)