[AppVeyor] AppVeyor upgraded their MySQL

appveyor.yml

@@ -10,7 +10,7 @@ environment:
         WODI: wodi32
         ADD_REVISION: 1
         OCAMLOPT: ocamlopt.opt
-        MYSQL_PATH: C:\Program Files\MySQL\MySQL Server 5.6
+        MYSQL_PATH: C:\Program Files\MySQL\MySQL Server 5.7
         MYSQL_USER: root
         MYSQL_PASSWORD: Password12!
     matrix: