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Go 1.8 doesn't have the `testing.Helper()` method
https://travis-ci.org/github/hub/jobs/481181084

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Closes #1781 [ci skip]

This is to be more explicit about the contribution process and license.

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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.

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

Fixes #1291

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Fixes #831 [ci skip]

Go 1.4.1 has some bug fixes for 1.4: https://github.com/golang/go/issues?q=milestone%3AGo1.4.1. Nothing is changed on our side.

As the Go version of hub matures, we plan to add features that are not
necessarily tied to git operations, such as viewing and managing issues.

Previously this wan't in the scope of the project. Now the scope widens.

- Ruby is not required at runtime anymore
- Most installation instructions are now obsolete. Will need to write
  new instructions when we distribute precompiled binaries.
- Remove tedious parts about slowness. It's not slow anymore!

Fixes #421