Make Util return timestamps in UTC, fixes #354

By using UTC timestamps, we can avoid a situation where a developer A in timezone 1
creates a migration A:1, and then developer B in an "earlier" timezone 2 creates a
migration B:2 that appears to be made before A:1.

- Creates a separate `Util::getCurrentTimestamp` method
- Adds a constant for `Util::DATE_FORMAT`
- `Util::mapClassNameToFileName` uses `Util::getCurrentTimestamp`