Point to a fork of lostisland/go-sawyer

Code changes from the latest master of lostisland/go-sawyer hasn’t been applied to octokit/go-octokit, causing compilation errors. The current approach of providing a wrapper script to bootstrap the library is confusing (see #62). It also makes using the library harder (we need to vendor to a particular commit, see https://github.com/github/hub/blob/gh/Godeps/Godeps.json#L57-L60).

Let’s point to the fork jingweno/go-sawyer for now which doesn’t include the latest breaking changes from lostisland/go-sawyer and unblock the mentioned issues.

/cc @technoweenie @pengwynn @mislav