Add and apply RSpec/BeNil rubocop rule

This rule was added in rubocop-rspec 2.9.0.

We were using `be_nil` 50% of the time, and `be nil` the rest of the
time. No strong preference for either one, but IMHO we don't lose
anything be being consistent.

spec/models/admin_notification_spec.rb

@@ -26,7 +26,7 @@ describe AdminNotification do
 
   describe "#complete_link_url" do
     it "does not change link if there is no value" do
-      expect(admin_notification.link).to be_nil
+      expect(admin_notification.link).to be nil
     end
 
     it "fixes a link without http://" do