snap: push is replaced with upload

[abitrolly]

Fixes deprecation warning

DEPRECATED: The 'push' set of commands have been replaced with 'upload'.
See http://snapcraft.io/docs/deprecation-notices/dn11 for more information.

https://travis-ci.org/github/yakshaveinc/linux/jobs/714166058#L397-L398

[abitrolly]

The last build error doesn't seem to relate to this PR.

bin/dpl scalingo --stage install --api_token str
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  1 8915k    1  151k    0     0  70278      0  0:02:09  0:00:02  0:02:07 70270
curl: (56) GnuTLS recv error (-54): Error in the pull function.