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 
On Fri, Sep 19, 2014 at 12:12 PM, Jingwen Owen Ou <notifications@github.com>
wrote:

> I'd suggest to migrate after the 2.0 is released along with the ini
> format changes
> <https://github.com/github/hub/pull/621#issuecomment-55606637>.
>

Alright!