Set API version from options in DynamoDB ctor.

1.9.3-p484 :069 > AWS::DynamoDB.new.client
 => #<AWS::DynamoDB::Client::V20111205> 
1.9.3-p484 :068 > AWS::DynamoDB.new( dynamo_db: { api_version: '2012-08-10' } ).client
 => #<AWS::DynamoDB::Client::V20120810>