Add a version number to the app.yml file

[macMikey]

Today, the app.yml file's format/structure was changed. The job of any developer who is building an automatic app.yml analyzer/updater would be easier if s/he didn't have to guess what version of the levure app.yml spec they are trying to read, and then which version they are trying to fit into.

[trevordevore]

This decision can be deferred until a version key is actually needed.