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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git-svn-id: http://core.svn.wordpress.org/trunk@30406 1a063a9b-81f0-0310-95a4-ce76da25c4cd