message: docs(Button): fix color prop version
diff: @@ -97,7 +97,7 @@ Vue.use(Button);
  | type | Can be set to `primary` `info` `warning` `danger` | *string* | `default` | - |
  | size | Can be set to `large` `small` `mini` | *string* | `normal` | - |
  | text | Text | *string* | - | - |
  -| color | Color, support linear-gradient | *string* | - | 2.1.3 |
  +| color | Color, support linear-gradient | *string* | - | 2.1.8 |
  | icon | Left Icon | *string* | - | - |
  | tag | HTML Tag | *string* | `button` | - |
  | native-type | Native Type Attribute | *string* | `''` | - |
  