Remove extra semicolon in Task.scala

Author: GuoQiang Li <witgo@qq.com>

Closes #1876 from witgo/remove_semicolon_in_Task_scala and squashes the following commits:

c6ea732 [GuoQiang Li] Remove extra semicolon in Task.scala

(cherry picked from commit 3570119c34ab8d61507e7703a171b742fb0957d4)
Signed-off-by: Reynold Xin <rxin@apache.org>