gh: Provide option to build `hub` without autoupdate

Use Go build tag to build with/without autoupdate. For Homebrew, we build with `./script/build noupdate` and let it handle the update. We could also build with `./script/build` which has autoupdate built-in.

/cc @mislav 
[Yes](http://golang.org/pkg/go/build/). Build tag is a powerful concept: strategy pattern at build time. In our context, we could also use it solve platform dependent features like [this](https://github.com/jingweno/gh/pull/191#issuecomment-34022250).