Fix broken home page due to missing `gerrit_path`

Commit f87a5b7dc253 renamed the `gerrit_path` method in app/routes.rb to
`gerrit_changes_path` but missed to change one reference to it.

Change-Id: If384462297f136629d6070aaa5fe7ac7439811e9
Reviewed-on: https://gerrit.causes.com/27011
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>