Use gerrit 2.8.5 instead of gerrit 2.6.1 in tests

Note: this change modifies .gerrit content (used by tests), so delete
.gerrit folder and run "python -m git_review.tests.prepare".

Change-Id: Ia12d7148e3127ed5e10da449f1f4eebe633262bd
Merge "Use gerrit 2.8.5 instead of gerrit 2.6.1 in tests"