gh: Fix `merge.feature` and `checkout.feature`

Updated go-sawyer, go-octokit and pflag

gh: Propagate authenticate error to the caller

This'll make sure we propagate error to the caller where we can reformat
the error message within the context.

This fixes `features/authentication.feature:85`

See comment https://github.com/github/hub/pull/515#discussion_r10915319

See https://github.com/jm/toml/issues/31

See https://github.com/octokit/octokit.rb/blob/ab0404e0f1c6d36ed52c6e9c8c9734a5f14ec839/lib/octokit/error.rb#L155

This reverts commit 26540bba.