Consider requiring Send on GetToken.token()s error type to ease use with Failure

[cpick]

The (fantastic!) downstream google* crates are slightly difficult to use with the failure crate ecosystem because their google_*::Error types don't automatically implement failure::Fail because they are !Sync and !Send because their google_*::Error:: MissingToken variant returns the error from yup_oauth2::GetToken::token(): Box<dyn std::error::Error>.

If yup_oauth2::GetToken::token()s error was also Send then google_*:Error would implement failure::Fail (it still would need to be Sync, but that's easily accomplished with the failure::SyncFailure helper).

Thanks for this useful set of crates!

[dermesser]

is this something you'd be comfortable creating a pull request for? It sounds like you're more familiar with what exactly needs to be done; I don't see a reason not to merge it :)