Polish specs for writing transactions

Author: andrykonchin

spec/dynamoid/transaction_write/execute_spec.rb

@@ -13,7 +13,7 @@
     klass.create_table
 
     expect {
-      transaction = described_class.execute do |t|
+      described_class.execute do |t|
         t.create klass
         t.create klass
       end