Update pre-selected theme to Twenty Fifteen

Twenty Twelve no longer seems to be included in the latest WordPress distribution. As it's the theme that's pre-selected in the provisioned database, you're greeted with a blank page when you first browse to `http://localhost:8080/`, as noted in #80.

This change just updates `template` and `stylesheet` in `wp_options` in the provisioned database dump to use the Twenty Fifteen theme instead.

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