Bump darkplaces version
    
    This bumps darkplaces version to 20150825. Failing to bump darkplaces
    version together with xonotic version bump leads to a situation where
    darkplaces _version_ stays the same, but _release_ goes backwards,
    essentially downgrading the package.
    
    This leads to a situation where people who already have an older version
    of xonotic + darkplaces installed, won't get the newer darkplaces
    snapshot because they already have one with higher release
    number installed.
    
    While at it, this commit makes the darkplaces dependency fully
    versioned, to make sure that the matching snapshot gets pulled in when
    installing xonotic.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1261993