remove rustc-serialize dependency

[ds-cbo]

Fixes #1073

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Sadly I wasn't able to run all tests locally because I got the following error while running them:

Error { kind: Tls, cause: Some(Error { code: -67843, message: "The certificate was not trusted." }) }

I am guessing this has something to do with macOS and Docker, but it's out of scope for me to debug that.

[ds-cbo]

Now that the clippy warnings have been fixed by the other PR, could you please retrigger CI for this one?

[sfackler]

You'll need to rebase/merge the master branch back into this branch.