Update column migration method to correctly set null: false

  * Update `MainAdapter#create_table` to call `add_spatial_column`

Edit:  Here is more specific information (taken from a comment that is now deleted):

Previously running 

```
create_table :my_table do |t|
  t.column :geometry, :geometry, null: false
end
```
would produce a table with the `geometry` column without `null: false`.  With this update, including the `null: false` flag will now set the column to be `null: false` as expected.