dynamodb: TestConditionalPutUpdateDeleteItem test fails against AWS

Hello everyone! I tired to run dynamodb tests against real AWS server and got this error:

```
$ go test -v -amazon -local=false
=== RUN Test

----------------------------------------------------------------------
FAIL: item_test.go:72: ItemSuite.TestConditionalPutUpdateDeleteItem

item_test.go:104:
    c.Check(err.Error(), check.Matches, "ConditionalCheckFailedException.*")
... value string = "ValidationException: One or more parameter values were invalid: Action ADD is not supported for the type S"
... regex string = "ConditionalCheckFailedException.*"

OOPS: 23 passed, 1 FAILED
--- FAIL: Test (203.87 seconds)
FAIL
exit status 1
FAIL	github.com/crowdmob/goamz/dynamodb	203.905s
```

This pull request is my naive approach to fix it: tests against local dynamodb fails, but pass on real dynamodb instance.