With the release of WordPress 3.7 you now can have your plugins and/or themes update happen behind the scenes.
To enable automatic updates for plugins, use the following:
add_filter( 'auto_update_plugin', '__return_true' );
To enable automatic updates for themes, use the following:
add_filter( 'auto_update_theme', '__return_true' );
More infomation available on the WordPress Codex
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