Bump used gerrit API to version 2.8-SNAPSHOT

Since current gerrit-plugin-api-2.8-SNAPSHOT does not come with
gerrit-pgm, we have to add it for now to be able to build. We can
remove that dependency again, once gerrit upstream includes gerrit-pgm
in gerrit-plugin-api again.

Change-Id: I58c3534765180bcc710c95449630095983729274