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.

.rubocop.yml

@@ -467,6 +467,10 @@ RSpec/AroundBlock:
 RSpec/BeforeAfterAll:
   Enabled: true
 
+RSpec/BeNil:
+  Enabled: true
+  EnforcedStyle: be
+
 RSpec/Capybara/CurrentPathExpectation:
   Enabled: true