Release v0.4.0

Summary: Bump version to v0.4.0.

Test Plan: Run `make && blubber --version` and verify the output.

Reviewers: thcipriani, #release-engineering-team

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D1060

VERSION

-0.3.0
+0.4.0