Fix SQS visibility timeout

Fix setting item visibility timeout (#131)

I am unsure why it happened now. Was it introduced in DispatchRider 2.0 or 2.1 ? 
Was it a change upstream in aws-sdk-sqs ?

* test using `ensure`, because item.visibility_timeout krash

* use change visibility

* make aws sns/sqs required runtime gem

* fix openstruct in specs

* use a proc

* require gems, do not rescue NameError exceptions

* bump version

* stub object for the method call instead

* rubocop