Add criterium back into the `:test` profile deps.

It turns out that I was wrong about criterium only being used in
development.

Q.v. [korma.test.connected](/test/korma/test/connected.clj).

(This commit is a partial spiritual reversion of bb73e9c42c0897a6a974.)
Restore note about requiring `java.jdbc` 0.2.3.

Until we've *released* a version of Korma that doesn't depend on
`java.jdbc`, the warning should probably remain in the [README](README.md).

This commit partially reverts 57897aa111badae123fc6df31f6f49c59cfb5ac3.
Don't advertise 0.3.0 yet; revert to 0.3.0-RC6.

I'll consider alternatives if issue #204 takes a while to resolve.