travis: add "overwrite: true" to releases provider

Possibly this is why we're getting breakage with GitHub releases.

Doing this on master as there's not a particularly great way to test the
pipeline otherwise, and it's incidental to user-facing code.

Author: brennen

.travis.yml

@@ -11,6 +11,7 @@ deploy:
   file_glob: true
   file: "$TRAVIS_BUILD_DIR/bundles/*"
   skip_cleanup: true
+  overwrite: true
   on:
     tags: true
 - provider: pypi