Fixed issue with auto incrementing primary key

Fixed an issue where creating a table with a column identified with `primary_key` would result in an SQL error. Reason being that vertica requires the type of an auto incrementing column to be `AUTO_INCREMENT` and not a data type. `AUTO_INCREMENT` also needs to have a underline between `AUTO` and `INCREMENT` which is not the default syntax that sequel used. 

For example the following create would fail

    @db.create_table(:foo) do
      primary_key :id
      Integer :bar
    end

This pr should correct the problem.