Bump the $wp_version.
This version string controls more than you might first imagine, but the important thing to note for now, is that this needs to contain a revision number in order for nightly builds to include a revision number.
If a nightly build doesn't include a revision number, Background Updates won't occur for Development users (after all, 4.1-beta1 == 4.1-beta1, which is already installed).
See #30405

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Use hash_equals() for old md5 hashes.

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