Update schema_diff.py to use 'postgresql' URLs.

This gets rid of a SQLAlchemy deprecation warning when using this tool w/
PostgreSQL:

The SQLAlchemy PostgreSQL dialect has been renamed from 'postgres' to
'postgresql'. The new URL format is
postgresql[+driver]://<user>:<pass>@<host>/<dbname>

This appears to have been deprecated in the SQLAlchemy 0.6 series so I think we
are Okay to remove it now.

Change-Id: If8fdf822ba452b387e9f2f3ec7c9157e4ae932f9
Merge "Update schema_diff.py to use 'postgresql' URLs."