Apply explict RSpec/DescribedClass rubocop rule

We settled on using this style in commit 4cbe81a1, but didn't add the
rule enforcing this style and we didn't apply it to existing code.

spec/models/verification/sms_spec.rb

@@ -8,7 +8,7 @@ describe Verification::Sms do
 
   it "validates uniqness of phone" do
     create(:user, confirmed_phone: "699999999")
-    sms = described_class.new(phone: "699999999")
+    sms = Verification::Sms.new(phone: "699999999")
     expect(sms).not_to be_valid
   end