Enables RSpec/ExampleWording and fixes all issues

Both avoiding 'should' and repiting 'it' on the tests description
improves reading them and also makes all descriptions consistent.

Read about cop at http://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/ExampleWording

spec/models/sms_spec.rb

@@ -1,12 +1,12 @@
 require 'rails_helper'
 
 describe Verification::Sms do
-  it "should be valid" do
+  it "is valid" do
     sms = build(:verification_sms)
     expect(sms).to be_valid
   end
 
-  it "should validate uniqness of phone" do
+  it "validates uniqness of phone" do
     create(:user, confirmed_phone: "699999999")
     sms = described_class.new(phone: "699999999")
     expect(sms).to_not be_valid