fix name for postgresql

[willgroves]

The code change in ef2670c caused tables on a postgres backend to lose the time grain selector in the table explorer (i.e. cannot see/select a time grain after this code change).

The previous code used startswith to match the database driver type (which was looking to match 'postgres' and 'postgresql').

https://github.com/airbnb/caravel/blob/8626c80d3a491b842f8262c5807154903c107747/caravel/models.py#L775

The new version requires matching the whole string so a postgresql:// uri will not match postgres.

https://github.com/airbnb/caravel/blob/ef2670ca32e8ba0a04f9dc7899db55812e2f9b46/caravel/models.py#L1365

The fix sets postgresql as the engine name. This corresponds to the sqlalchemy doc as well.

[mistercrunch]

thanks for the fix!