Remove unnecessary variable assignment

We used to dump/restore the test database ourselves,
but later moved to using Rails' structure.sql or
the db:test:prepare rake task.
Gracefully handle scenario where neither structure.sql nor Rakefile exist

This is useful in non-Rails usage scenarios, in the future,
if we wanted to support non-Rails test databases, we could fall
back to manually dumping/restoring the test database defined
in the database.yml config.
Update README to include non-Rails usage instructions