Make commit author and committer optional

[Sega-Zero]

Noticed that in some cases commits may loose author (when they pushed with the wrong email, for example).

had this in commits response and danger-swift failed to parse it:

{
    "sha": "124c29519f7bf5427f7898c321dad561e2443f36",
    "commit": {
      "author": {
        "name": "User",
        "email": "user@mail.com",
        "date": "2018-05-08T08:22:20Z"
      },
      "committer": {
        "name": "User",
        "email": "user@mail.com",
        "date": "2018-05-08T08:22:20Z"
      },
      "message": "...",
      "tree": {
        "sha": "206f9b31716726968de6a35c2ee19a1ada62e8e5",
        "url": "https://api.github.com/repos/company/repo/git/trees/206f9b31716726968de6a35c2ee19a1ada62e8e5"
      },
      "url": "https://api.github.com/repos/company/repo/git/commits/124c29519f7bf5427f7898c321dad561e2443f36",
      "comment_count": 0,
      "verification": {
        "verified": false,
        "reason": "unsigned",
        "signature": null,
        "payload": null
      }
    },
    "url": "https://api.github.com/repos/company/repo/commits/124c29519f7bf5427f7898c321dad561e2443f36",
    "html_url": "https://github.com/company/repo/commit/124c29519f7bf5427f7898c321dad561e2443f36",
    "comments_url": "https://api.github.com/repos/company/repo/commits/124c29519f7bf5427f7898c321dad561e2443f36/comments",
    "author": null,
    "committer": null,
    "parents": [
      {
        "sha": "9ecf478dbc6c95e2001642cc988b4e64eb4cea72",
        "url": "https://api.github.com/repos/company/repo/commits/9ecf478dbc6c95e2001642cc988b4e64eb4cea72",
        "html_url": "https://github.com/company/repo/commit/9ecf478dbc6c95e2001642cc988b4e64eb4cea72"
      }
    ]
  }

Just like #73 it's somehow related to #68 :)

[DangerCI]

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[orta]

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[peril-staging]

Thanks for the PR @Sega-Zero.

This PR has been shipped in v0.4.1 - CHANGELOG.