Update roadmap to next planned Gerrit version

Gerrit 3.2 is already released. Hence, clean up and update the
roadmap to mention plans for Gerrit 3.3.

If you want to know what ended up being implemented in 3.2, please
take a look at the release notes.

Change-Id: I033242c20147bccacba713d6b8752d135d8076c0
1 file changed