travis: Set deploy.skip_cleanup to true.

As described in https://docs.travis-ci.com/user/deployment/releases/

Hopefully this will get deployment to github releases working again.
Though I don't quite understand how it ever worked before without it,
nor why isn't this the default already.

.travis.yml

@@ -65,6 +65,7 @@ deploy:
  secure: frTz8L+ex+foqv1fpVJX9guW/YHftxhjJ3SKwcfLmIW/NP77FVLgF8PgyLyPVbRkUOoSZpNYGQcWEpOF3bEK5HbeQScuV75TgSO2n2Ie4UqMO7+4G3bY5+Ni/SvfA/qt7krZQeNc0wCtPX+QjD4HLmSXfnie6PMZw/q5pkjN9fI=
  file_glob: true
  file: build/drmingw-*.zip
+ skip_cleanup: true
  on:
  repo: jrfonseca/drmingw
  tags: true