Duplicated key *url* in meta.yaml

[msviridov-rocket]

When trying to process meta.yaml using ruamel_yaml it shows a error. It can be suppressed now, but it will become a real error in future. If you want to use several sources you have to use a list like that:

source:
  - url: <first url here>
    sha256: <checksum for the url above>
  - url: <second url here>

Otherwise YAML parsers raise DuplicatedKeyError or another exception with the same meaning.

[TomAugspurger]

Do you want to submit a PR removing the first url? The GitHub one is the correct one.