Log Message:
Update contao31 package to 3.1.5.

Version 3.1.5 (2013-11-08)
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### Fixed
Correctly handle shorthand byte values (see #6345).

### Fixed
Also update the sitemap if a news/event feed is updated (see #5727).

### Fixed
Correctly sort by date in the listing module (see #5609).

### Fixed
Correctly handle the autologin key if a member is duplicated (see #5945).

### Fixed
Correctly export pages as PDF (see #6317).