`elasticsearch_version` 2.3
Fixes#7836
This will allow ElasticSearch domains to be deployed with version 2.3 of
ElasticSearch
The other slight modifications are to stop dereferencing values before
passing to d.Set in the Read func. It is safer to pass the pointer to
d.Set and allow that to dereference if there is a value
```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSElasticSearchDomain_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSElasticSearchDomain_ -timeout 120m
=== RUN TestAccAWSElasticSearchDomain_basic
--- PASS: TestAccAWSElasticSearchDomain_basic (1611.74s)
=== RUN TestAccAWSElasticSearchDomain_v23
--- PASS: TestAccAWSElasticSearchDomain_v23 (1898.80s)
=== RUN TestAccAWSElasticSearchDomain_complex
--- PASS: TestAccAWSElasticSearchDomain_complex (1802.44s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/aws 5313.006s
```
Update resource_aws_elasticsearch_domain.go
George Christou