Add version that properties were introduced in (vuejs#1914)

`$attrs` and `$listeners` were introduced in v2.4.0. The docs should mention this in order to avoid confusion.

Author: d-levin

src/v2/api/index.md

@@ -1430,6 +1430,8 @@ type: api
 
 ### vm.$attrs
 
+> New in 2.4.0+
+
 - **Type:** `{ [key: string]: string }`
 
 - **Read only**
@@ -1440,6 +1442,8 @@ type: api
 
 ### vm.$listeners
 
+> New in 2.4.0+
+
 - **Type:** `{ [key: string]: Function | Array<Function> }`
 
 - **Read only**