Ability to change DSNs after calling `uninstall`

Today you can't change the DSN of an already-configured Raven instance, even if you call `uninstall` on it.

Via support:

> We have sentry projects for QA and for production, the reason I want to switch DSN is that in some scenarios we want to send massages to QA and not production (to "sterilize" production project), and those scenarios can be during runtime.
