The only thing that has substansively changed is that Go 1.8 handles
redirect logic more safely than previous versions. This means we can
drop our special handling to avoid following redirects to other
domains. We were only doing that to protect against the possibility
of leaking auth headers. With Go 1.8, the auth headers are not
forwarded when following a redirect to another domain, so we don't
need our special handling any more.

As long as people are attempting to build with the Makefile, the new
check_go_version script should cause the build to stop if our
collaborators aren't using at least version 1.8 of go.

The current functionality should really be moved to "hub pr checkout".
This is a step on the way.

It wraps functionality already provided by "hub checkout". The advantage
with "hub review" are twofold:

* Discoverability. It wasn't until I started browsing the hub source
  code that I realized this functionality was present.
* Usability. Just typing the PR number is easier than having to figure
  out its URL.

Installs hub binary and man pages into `/usr/local` by default.

Fixes #1291

Ported from my bash version over at
https://github.com/mislav/dotfiles/blob/370154a5adc482afd89606c126bcd18df2560013/bin/git-sync

Old `script/bootstrap` & `script/build` were unfriendly to Go newbies
because they require the user having GOPATH set in their environment,
and either `godep` installed or `hg` to be able to fetch godep.

However, since dependencies are vendored, we don't have any real build
dependencies except Go itself.

- `script/bootstrap` now checks Go and installs Ruby test bundle
- `script/build` skips compiling if binary is up to date
- `script/test` runs both Go and Cucumber test suites

Advantages over scripts in `script/`:

- Works without gopath
- Works without godep