Ignore num_no_build column of versions.csv#41

The **num_no_build** column is identical to **num** but without build metadata. So if **num**="1.0.0+example" then **num_no_build**="1.0.0". In Rust code, there is no reason to deserialize the value of this column because it is easily reconstructed from **num**. It exists in the database only for the purpose of defining a `unique` constraint on `(crate_id, num_no_build)`.