Account for clock drift in `exp`

[dscho]

See this: https://github.com/git-for-windows/git-for-windows-automation/blob/a8955675901a77b42853ef36fe13722a9a657fa1/github-api-request-as-app.js#L9-L16

The exp property "must be no more than 10 minutes into the future", but we'll probably need to account for clock drift here, too. GitHub recommends to set the iat property "60 seconds in the past", "to protect against clock drift", but exp probably needs some protection, too.

Originally posted by @rimrul in git-for-windows/MINGW-packages#175 (comment)