Update Swagger scaling edpoint docs to POST

The scaling endpoint supports POST method, but in Swagger it was
documented with GET instead. Also bumped up the gateway version
to 0.8.12

Signed-off-by: Ivana Yovcheva (VMware) <iyovcheva@vmware.com>

api-docs/swagger.yml

 swagger: '2.0'
 info:
   description: OpenFaaS API documentation
-  version: 0.8.9
+  version: 0.8.12
   title: OpenFaaS API Gateway
   license:
     name: MIT
@@ -209,7 +209,7 @@ paths:
         '500':
           description: Internal server error
   '/system/scale-function/{functionName}':
-    get:
+    post:
       summary: Scale a function
       parameters:
       - in: path