Snap stable channel not updating

[casperdcl]

Edge (master) releases are deploying correctly, but stable (tagged) releases aren't

[casperdcl]

The stable snap deploy still didn't quite work after #3158.

• TRAVIS_TAG undefined => edge
  • https://travis-ci.com/iterative/dvc/jobs/277406494#L2150-L2164
• TRAVIS_TAG defined => stable$TRAVIS_TAG rather than stable
  • https://travis-ci.com/iterative/dvc/jobs/277406823#L2162-L2170

on the plus side everything else worked. Also, this should be easy to fix.