Fixed test warnings [skip ci]

Rakefile

@@ -4,7 +4,7 @@ require "rake/testtask"
 Rake::TestTask.new(:test) do |t|
   t.libs << "test"
   t.test_files = FileList["test/**/*_test.rb"]
-  t.warning = false
+  t.warning = false # for carrierwave and shrine
 end
 
 task default: :test

test/model_test.rb

@@ -660,7 +660,7 @@ def test_previous_versions_key
     email = "test@example.org"
     key = User.lockbox_attributes[:email][:previous_versions][0].fetch(:key)
     box = Lockbox.new(key: key, encode: true)
-    user = User.create!(email_ciphertext: box.encrypt(email))
+    User.create!(email_ciphertext: box.encrypt(email))
     assert_equal email, User.last.email
   end
 
@@ -669,7 +669,7 @@ def test_previous_versions_master_key
     master_key = User.lockbox_attributes[:email][:previous_versions][1].fetch(:master_key)
     key = Lockbox.attribute_key(table: "users", attribute: "email_ciphertext", master_key: master_key)
     box = Lockbox.new(key: key, encode: true)
-    user = User.create!(email_ciphertext: box.encrypt(email))
+    User.create!(email_ciphertext: box.encrypt(email))
     assert_equal email, User.last.email
   end