Small fixes and v5.3.0 prep (Unleash#219)

* Add request timeout to cache bootstrap from URL.

* Only send Slack messages if not a fork (so CI doesn't fail for contributers).

* Update README.md.

Update README for v5.2.0.

Change logging if no feature flags are returned from server (Unleash#200

)

* Reduce level and reword message if no features are returned from server (whether 304 or error).

* Update changelog.

Fix logging in UCP. (Unleash#198)

Update changelog. (Unleash#196)

Release v5.0.1 (Unleash#187)

* Update changelog.

* Automatic version bump via bumpversion.

Bump version: 5.0.0 → 5.0.1

Bump version: 4.4.1 → 5.0.0

Bump version: 4.4.0 → 4.4.1

Bump version: 4.3.0 → 4.4.0