Correctly mark allowed failures for non-Ruby projects

We use `Job#matrix_config?` to test if an `allow_failures` entry matches a job, which gets the keys to compare from `Build.matrix_keys_for`. Since the `allow_failures` entry doesn't include a `language` key, `Build.matrix_lang_keys` only returns the keys included that are "Ruby" keys, since Ruby is the default language. This in turn makes the config that `matrix_config?` tests against empty, causing no jobs to be marked as allowed failures.