Reject Ace modes that are lacking a mode

@aroben suggested that the  `ace_modes` API only return valid entries, and I agree. This makes iterating over the list easier. 

/cc @github/languages 
You know: what if instead of `none`, the YAML file just states `text` for languages missing an Ace mode? This applies just a plaintext mode, with not highlighting whatsoever. Not sure why I didn't think of this before, but it would eliminate the need to introduce this other syntax, and avoids the need to filter out empty values later on.