Allow for .yaml extension of appspec

According to this one:

https://stackoverflow.com/questions/21059124/is-it-yaml-or-yml/21059164

.yaml is the preferred extension to .yml

I think it does make sense that appspec could be appspec.yml AND appspec.yaml. A current codeDeploy task with appspec.yaml instead of appspec.yml fails with:

`The CodeDeploy agent did not find an AppSpec file within the unpacked revision directory at revision-relative path "appspec.yml". The revision was unpacked to directory
....
...
...`

The proper approach should be to allow for the deprecated .yml format but also to allow for .yaml file as that's the recommended extension.