submarine:Randomise rock distance for latter level

Previously, the distance of the rock1 from the crown was same, leading
to the fact that the difficulty of the levels from level 7 to 10 was
almost the same. To overcome this, after the 7th level, the distance of
rock1 and the crown was randomised to add to the difficulty

Signed-off-by: Rudra Nil Basu <rudra.nil.basu.1996@gmail.com>