Previously, `rake spec` ran the tests whether or not the `growstuff_test`
database existed or not, but `rake` crashed unless it had already been
created. The problem was that `rake` was trying to load the environment,
which required the presence of a DB; `rake spec` was running this with
the `dev` environment, so succeeded if `growstuff_dev` existed, but
`rake` was running everything with the `test` environment, so would fail
if `growstuff_test` did not exist.
This patch adds `db:create` to the dependencies of the `spec` task,
forcing an unadorned `rake` to create all necessary databases first.
