Remove duplicate line "sudo: false" in .travis.yml

sudo: false has already been added in 4fd22bb88aeef1385ce9d057f46cedfac07b569a.

.travis.yml

@@ -24,4 +24,3 @@ notifications:
       - "chat.freenode.net#celery"
     on_success: change
     on_failure: change
-sudo: false