Fix deprecation in specs

DEPRECATION: `expect { }.not_to raise_error(SpecificErrorClass)` is deprecated. Use `expect { }.not_to raise_error` (with no args) instead. Called from /home/travis/build/gitlabhq/gitlabhq/spec/tasks/gitlab/backup_rake_spec.rb:42:in `block (4 levels) in <top (required)>'.