Validate lft and rgt should be less then max sequence

[sampatbadhe]

Validate lft and rgt should be less then max sequence which is (number of records * 2)

i.e.

category1 = { parent_id: nil, lft: 1, rgt: 2 }
category2 = { parent_id: nil, lft: 3, rgt: 4 }
category3 = { parent_id: nil, lft: 9, rgt: 10 }

In above case, Category.valid? returns true, but expecting to return false so that it can rebuild again to correct the sequence.

Please feel free to close this PR if my changes looks invalid, as I noticed the scenario which I mentioned was considered as valid case and covered in spec that way.