Update go get for stable version

In the future, github default branch will be develop so running `go get github.com/gin-gonic/gin` will pull latest code from develop.

README.md

@@ -88,13 +88,13 @@ BenchmarkZeus_GithubAll 		| 2000 		| 944234 	| 300688 	| 2648
 1. Download and install it:
 
     ```sh
-    $ go get github.com/gin-gonic/gin
+    $ go get gopkg.in/gin-gonic/gin.v1
     ```
 
 2. Import it in your code:
 
     ```go
-    import "github.com/gin-gonic/gin"
+    import "gopkg.in/gin-gonic/gin.v1"
     ```
 
 3. (Optional) Import `net/http`. This is required for example if using constants such as `http.StatusOK`.