`.jay` incompatibility

There is an issue caused by PR https://github.com/h2oai/datatable/pull/3144. All the `.jay` files produced by datatable as of this PR are causing segfaults, when read by any of the older datatable versions. For instance, doing

```python
from datatable import dt
DT = dt.Frame([1])
DT.to_jay("one.jay")
```
in datatable `1.1.0a2081+` and then doing
```python
from datatable import dt
DT = dt.Frame("one.jay") #segfault
```
results in a segfault in any datatable version prior to `1.1.0a2081` .