Allow building of EC2 instances with ebs_optimized flag

Allows building of instances that are EBS optimized.

Depends on pull request in mitchellh/goamz:
https://github.com/mitchellh/goamz/pull/94

I didn't write any tests because this is my first Go code and although I had a go at working out how the tests were supposed to be written, it wasn't clear to me. Happy to write a test with some guidance!

I did a manual test and the flag was set on the instance and is shown in the .tfstate file:
```
jonathan@nomad $ ./bin/terraform show terraform.tfstate 
aws_instance.example:
  id = i-cc722af2
  ami = ami-7798074d
  availability_zone = ap-southeast-2b
  ebs_optimized = true
  instance_type = c3.xlarge
  key_name = 
  private_dns = ip-172-31-7-7.ap-southeast-2.compute.internal
  private_ip = 172.31.7.7
  public_dns = ec2-54-206-73-128.ap-southeast-2.compute.amazonaws.com
  public_ip = 54.206.73.128
  security_groups.# = 1
  security_groups.0 = default
  subnet_id = subnet-713d3413
```