Fix default theme not being copied on dev/build

[sminnee]

From http://open.silverstripe.org/ticket/5954

A default theme "blackcandymobile" exists in the mobile module, which is available as a base to start a mobile site.

dev/build will attempt to copy this theme into the SilverStripe? project's theme folder through the use of DataObject::requireDefaultRecords(), provided that it doesn't exist, or if another theme is being used for the mobile theme.

However, this doesn't occur. When installing the module the user has to copy the blackcandymobile folder from the module folder to the themes manually if they would like to use it. This would also be true if write permissions to the themes directory aren't available, which may be true for many cases anyway.