Update stack.yml

When using the current stack, Cloudformation throws an error "AMI ami-e4dde5a1 is invalid: The image id '[ami-e4dde5a1]' does not exist".  Looks like the east-1 region was using the id from us-west-1.
apply us-east-1 ami to stack.json
Merge pull request #2 from jhmartin/patch-1

Update stack.yml with correct us-east-1 ami.