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/setting_spec.rb

@@ -250,7 +250,7 @@ describe Setting do
 
         Setting.add_new_settings
 
-        expect(Setting.find_by(key: :stub).value).to be_nil
+        expect(Setting.find_by(key: :stub).value).to be nil
       end
     end
 
@@ -265,7 +265,7 @@ describe Setting do
         Setting.add_new_settings
 
         expect(Setting.where(key: :stub)).not_to be_empty
-        expect(Setting.find_by(key: :stub).value).to be_nil
+        expect(Setting.find_by(key: :stub).value).to be nil
       end
 
       it "doesn't modify custom values" do