add `database_version` to the connection object

This method was implemented in the Rails fork, but I think it's safe to
move over to the application as a monkey patch.  We'll extract it to a
gem later.

This implementation differs from the original in that it doesn't cache
the value of the database version.  I think this is OK for two reasons:

1. It's not a hotspot (that I can tell) so there shouldn't be a perf
impact.

2. The cached version could be wrong if we disconnect then reconnect.